Including needed boost::posix_time.h

[menaczar]

I'm getting a compiler error when trying the gigasecond exercise.
The library needed for posit_time was missing.
Please check the boost documentations: http://www.boost.org/doc/libs/1_59_0/doc/html/date_time/posix_time.html

[arcuru]

This has come up before in #142. The missing header is by design, you need to include it in your header file anyways to pick up the appropriate types.

Thanks

[arcuru]

Nvm, after looking at how many open issues we have around this I've changed my mind.

I've been against this in the past (since you have to put in real effort to write an implementation that actually needs this include in the test suite), but given how many issues we've seen with people hitting initial compile problems withmeetup and gigasecond (See #201, #142, #149, #150) I'm going to stop being stubborn and just merge it.

Thanks