Changes to clear warnings from MSVC 16.x

[bhardwajs]

Fixes #1911

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• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
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Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for these kind of bugs). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[bhardwajs]

@nlohmann - Travis CI failed for XCode 9.3. It seems a transient failure. Is it possible to retry just that compiler? It's succeeded on other XCode versions.

[dota17]

Is it similar to #1967 ?

[bhardwajs]

@nlohmann - Let me know if you need anything from me for this PR. Thanks!

[nlohmann]

Can you please update to the latest develop branch? It seems that this issue is already solved.