Cut GCC 5.0 preprocessor guard in Traits.h (#1151)
Cut GCC 5.0 preprocessor guard in Traits.h (#1151)

Summary:
- Cut `#if __GNUC_PREREQ(5, 0)` preprocessor directive as folly does not
  support any GCC version lower than 5.1.
- Change from `FOLLY_GNU_DISABLE_WARNING` to `FOLLY_GCC_DISABLE_WARNING`
  since Clang does not support `-Wbool-compare` option.
Pull Request resolved: https://github.com/facebook/folly/pull/1151

Reviewed By: Orvid

Differential Revision: D15618528

Pulled By: yfeldblum

fbshipit-source-id: 039e0aad617ccb40392505a6f4585e93d52dc81c

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly