Restrict Travis builds to the master branch

This avoids duplicated builds when using feature branches in the same repo to send pull requests
Merge pull request #3 from stof/patch-1

Restrict Travis builds to the master branch
Fixed the skipping of the test based on phpt expectations
Fix another test skipping
Merge pull request #5 from stof/patch-2

Fixed the skipping of the test based on phpt expectations
Update README.md
Merge remote-tracking branch 'origin/master' into garbage_collection