More tests running on Travis, lots of bugfixes and some new features as well

Changes from beta 6 include:

• better error messages
• references are allowed to select a contentType to update or delete
• contents to be updated/delete can be identified simply by their contentType identifier. This makes it easy to clean the db before removing the contentType itself
• migrations definitions in files available anywhere on disk can now be added to the migrations table
• plenty of bug fixes

For the complete list of changes since version 1.x, have a look at https://github.com/kaliop-uk/ezmigrationbundle/blob/master/WHATSNEW.md