Added support for coveralls.io
Added Coveralls badge for the readme.

[ci skip]
Merge pull request #2682 from refinery/coveralls

Added support for coveralls.io
Switched to SVG badges for the readme.

[ci skip]
Merge pull request #2683 from refinery/svg-badges

Switched to SVG badges for the readme.
Updated Rails version support in readme.

Refinery 3.0.0 will only work on Rails 4.1+

Also fixed the branch for coveralls.

[ci skip]
Changed wording, we don't need to say Rails twice.

[ci skip]