runkit: Change git from git:// to https://
runkit: Provide alternative repository for PHP 5.6.
Merge pull request #464 from bantu/runkit-php5.6

Restore runkit on Travis CI PHP 5.6.

* bantu/runkit-php5.6:
  runkit: Provide alternative repository for PHP 5.6.
  runkit: Change git from git:// to https://