Change datamapper references to rom-rb

This updates the status icon links in the README for travis and Gemnasium,
the install instructions, and the homepage in the gemspec.

The icon/link to CodeClimate is not updated yet because CodeClimate is seemingly
still unaware of 'rom-rb/devtools'.
Change references in the README from dm-2 to ROM
Merge pull request #18 from plexus/name_change

Change datamapper references to rom-rb