Changed framework dependency to dev-master

3.2.x-dev breaks composer constraint checks
because aliases aren't correctly resolved
when we require framework as follows:
"silverstripe\/framework": "dev-pulls\/lastvisited as dev-master".
Example build failure: https://travis-ci.org/chillu/silverstripe-framework/jobs/12311113

This should be fixed in composer itself, but for now its an easy workaround.
Once we branch off 3.2, we need to change the dependency declaration.