Ci fix after moving to cpp17 (#455)

* Update travis configuration.

* Update GCC_VER in travis configuration.

* Leave only Visual Studio 15 in Appveyor.

* Travis - remove GCC_VER 4.9.

* Travis - update clang-xcode.

* Travis - update clang compiler.

* Travis - downgrade clang compiler to 3.4.

* Revert "Travis - downgrade clang compiler to 3.4."

This reverts commit bd6698bccea9252dd00e8f28fb31334b0e3ea743.

* Travis - clang-6.0 package and compiler.

* Travis - remove (perhaps unnecessary) compiler option.

* Travis - restore clang compiler option.

* Update .travis.yml

* Another attempt to fix Travis config for clang.

* Another attempt to fix Travis config for clang.

* Compiler package for clang.

* Compiler package for clang.

* Xcode 10.

* Remove xcode from equation.

* Force install clang-6.0.

* Does it install these packages at all???.

* Does it install these packages at all???.

* Some appveyor fixes.

* Update CMakeLists.txt

Enforce C++17 standard

* Update .travis.yml

Restore xcode, block CLANG_VER.

* experimental/variant for Apple compilers.

* Remove OSX pipeline.

* Remove OSX pipeline.

* Attempt at fixing AppVeyor pipeline.

* Restore proper VS version in AppVeyor configuration.

* Revert preprocessor changes in json.hpp.

.travis.yml

@@ -5,10 +5,12 @@ addons:
   apt:
     sources: 
       - ubuntu-toolchain-r-test
+      - llvm-toolchain-trusty-5.0
+      - sourceline: 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-5.0 main'
+        key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
     packages:
-      - g++-4.9
-      - g++-5
-      - g++-6
+      - g++-7
+      - g++-8
   coverity_scan:
     project:
       name: "ChaiScript/ChaiScript"
@@ -22,7 +24,7 @@ matrix:
   include:
     - os: linux
       sudo: false
-      env: GCC_VER="4.9"
+      env: GCC_VER="7"
       compiler: gcc
       #    - os: linux
       #sudo: false
@@ -30,19 +32,20 @@ matrix:
       #compiler: gcc
     - os: linux
       sudo: false
-      env: GCC_VER="5" CPPCHECK=1 CMAKE_OPTIONS="-D RUN_FUZZY_TESTS:BOOL=TRUE"
+      env: GCC_VER="7" CPPCHECK=1 CMAKE_OPTIONS="-D RUN_FUZZY_TESTS:BOOL=TRUE"
       compiler: gcc
     - os: linux
       sudo: false
-      env: GCC_VER="6" CPPCHECK=1 COVERAGE=1 CMAKE_OPTIONS="-D RUN_FUZZY_TESTS:BOOL=TRUE"
+      env: GCC_VER="8" CPPCHECK=1 COVERAGE=1 CMAKE_OPTIONS="-D RUN_FUZZY_TESTS:BOOL=TRUE"
       compiler: gcc
-    - os: osx
-      compiler: clang
-      osx_image: xcode8
-    - os: osx
-      compiler: clang
-      osx_image: xcode8
-      env: CMAKE_OPTIONS="-D DYNLOAD_ENABLED:BOOL=FALSE -D MULTITHREAD_SUPPORT_ENABLED:BOOL=FALSE -D USE_STD_MAKE_SHARED:BOOL=TRUE" BUILD_ONLY=1
+    #- os: osx
+    #  compiler: clang
+    #  osx_image: xcode10
+    #  env: CLANG_VER="5.0"
+    #- os: osx
+    #  compiler: clang
+    #  osx_image: xcode10
+    #  env: CLANG_VER="5.0" CMAKE_OPTIONS="-D DYNLOAD_ENABLED:BOOL=FALSE -D MULTITHREAD_SUPPORT_ENABLED:BOOL=FALSE -D USE_STD_MAKE_SHARED:BOOL=TRUE" BUILD_ONLY=1
   
 env:
   global:
@@ -52,6 +55,7 @@ env:
 
 before_install:
   - if [ "${GCC_VER}" != "" ]; then export CXX="g++-$GCC_VER" CC="gcc-$GCC_VER" GCOV="gcov-$GCC_VER" ; fi
+  #- if [ "${CLANG_VER}" != "" ]; then export CXX="clang++-$CLANG_VER"; fi
   - pip install --user cpp-coveralls
 
 script: 

CMakeLists.txt

@@ -4,6 +4,9 @@ if(NOT ${CMAKE_VERSION} VERSION_LESS "3.1")
   cmake_policy(SET CMP0054 NEW)
 endif()
 
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
 project(chaiscript)
 
 option(MULTITHREAD_SUPPORT_ENABLED "Multithreaded Support Enabled" TRUE)

appveyor.yml

@@ -1,10 +1,9 @@
 version: 6.1.x.{build}
-image: 
- - Visual Studio 2017
+image:
+  - Visual Studio 2017
 environment:
   matrix:
-    - VS_VERSION: "Visual Studio 14"
-    - VS_VERSION: "Visual Studio 15"
+  - VS_VERSION: "Visual Studio 15"
 build_script:
 - cmd: >-
     mkdir build