Relaxed dependencies for future Mongoid 4.0 support

Helps with #94
attach a first file in an existing embedded document
Merge pull request #95 from locomotivecms/master

Fix a bug with embedded documents
Updated spec for readability

Ref #95
Bumped version to 0.6.0
Merge remote-tracking branch 'upstream/master'

Conflicts:
	carrierwave-mongoid.gemspec