Don't expose git_hash_ctx since it's internal

And doing so makes the mingw build choke.
travis: exit on failure for anything related to building
Merge pull request #1797 from ethomson/keep_hash_ctx_private

Don't expose git_hash_ctx since it's internal
Use time(2) to get the time

We didn't use the added precision in gettimeofday, so remove it.
This prevents us from having an unnecessary reimplementation on
win32.
Merge pull request #1799 from ethomson/gettimeofday

Use time(2) to get the time