Enforce dav4rack verion in Gemfile

Existing DMSF installations using older versions of dav4rack will
not install the appropriate version when bundle install is called.

Require version 0.3.0 of the gem as it contains the Utils name
space, not present in 0.2.11.
Merge pull request #306 from sempervictus/bug_dav4rack_version

Enforce dav4rack verion in Gemfile