Taken from
https://github.com/travis-ci/docs-travis-ci-com/pull/746/files.

To test a fix for issue #464 (not yet implemented), we first need to
have  an MSVC build with “/permissive- /std:c++latest /utf-8”.

We totally forgot to implement the comparison operators other than ==
and != for scalar types. Consequently, comparing a JSON value with a
scalar type led to compile errors.

add enum class default conversions

Properly pop diagnostics

The implementation is non-standard. Deprecation allows a simpler API in
the future without removing any features.

NaN values never compare equal to themselves or to other NaN values.

https://blog.travis-ci.com/2017-03-28-xcode-83-ga-is-here

We now only demand our exceptions to be is_nothrow_copy_constructible
if std::runtime_exception is.

The solution with a std::runtime_error member is more elegant. It
allows to have std::exception as base class again. However, I still
have no idea why GCC thinks the copy constructor may throw...

To have nothrow-copy-constructible exceptions, we inherit from
std::runtime_error which can cope with arbitrary-length error messages.
Intermediate strings are built with static functions and then passed to
the actual constructor.

Add Visual Studio 17 image to appveyor build matrix

UTF8 encoding enhancement

Fix typo