There seems to be some upstream bit rot with Clang++ 3.3. This is a
pretty old version, and we may want to just stop testing on it. However,
in the meantime, it will be allowed to fail without failing the overall
build.
Resolves#63.
[skip appveyor]
Until recently, CMake reported VS2015 as not supporting constexpr.
However, constexpr support was added in the VS2015 release, and CMake
now reports accordingly.
The constexpr support in VS2015 is too buggy to build Better Enums.
This change disables testing on VS2015 with constexpr support, because
those tests will fail.
Also adapted to a change in the Cygwin path prefix in AppVeyor and
fixed an issue in the Travis build with wget being unable to retrieve
from www.cmake.org's new redirect to cmake.org.
Also reordered compilers so that the very first tested are the ones
that support the most configurations, then immediately followed by
those which are the most likely to fail. Typically, this would be the
oldest ones, or the compiler versions that were the first to support
some major features.
