Adds failing feature spec for element trashing. Related to #506
Uses correct DOM id for removing tinymce instance. Fixes #506
Raises poltergeist timeout from 30 to 60sec for CI env.

In some cases (in a slow CI environment) 30seconds of waiting is not enough and poltergeist raises a TimeoutError. This commit raises this limit to 60sec.
Removes db connection sharing between threads and adds database_cleaner.

This runs much more stable, also when running in different environments like slow CI servers.
Bump version to 2.7.4