Migrate to C++14

[baylesj]

[dota17]

The main new features in C++14 is:

1. Return type deduction

2. Generic lambdas

3. Initialized lambda captures

4. Relaxed constexpr restrictions

5. [[deprecated]] attribute

6. Digit separators and Binary literals

Can we apply the 6th feature to the ValueTest for a new unittest case?

[hjmjohnson]

I am a bit concerned about moving to C++14. Many projects that will be working with jsoncpp have not yet migrated to C++14 development environment, and this change will make it difficult to incorporate jsoncpp (thus limiting its widespread adoption).

[dota17]

Migrate to C++14:

• Delete JSONCPP_DEPRECATED, use [[deprecated]] instead : Delete JSONCPP_DEPRECATED, use [[deprecated]] instead. #978

[cdunn2001]

We could have a C++14 branch. Hopefully, after the clang-format changes, there will not be any more large changes. In general, we would need changes on both pre- and post-C++14 branches. Not fun, but possible.

[baylesj]

Let's rereview this in the future, then. I don't think the jump between 11 and 14 is great enough to warrant the suffering.