Disabled APC on Travis for PHP 5.5+ as it is not available
merged branch stof/fix_travis_5_5 (PR #7663)

This PR was merged into the 2.1 branch.

Discussion
----------

Disabled APC on Travis for PHP 5.5+ as it is not available

As APC is not available, PHP triggers a warning when trying to load the extension, which lead to many test failures.

Commits
-------

6084176 Disabled APC on Travis for PHP 5.5+ as it is not available