Fix Travis build.

- Run `make vendor` early to avoid Composer clashing with APC.
- Continue skipping some JSON tests on HHVM 3.3.1 since not all the bugs
  are fixed yet.
- Test on PHP 5.6
Fix Travis build.

- Run `make vendor` early to avoid Composer clashing with APC.
- Continue skipping some JSON tests on HHVM 3.3.1 since not all the bugs
  are fixed yet.
- Test on PHP 5.6
Update dev dependencies
Add some logging to the caching.

In the process, streamline some of the caching-related code.