Fix EZP-19923: Missing config for Null type ezsubtreesubscription causes exceptions
Fix EZP-20434: Do not try to delete binary files with empty path

Follow up to 1218b113709fe44c54022f91400c741c71cbd189.
Merge pull request #239 from ezsystems/EZP20434-empty-path

Fix EZP-20434: Do not try to delete binary files with empty path
Removed composer selfupdate from .travis.xml, travis is updated
Change .travis.yml composer command to prefer-source to try to avoid timeouts