Remove datamapper specific version 1.0.0

For Ruby2.0.0-rc1
cannot load such file -- iconv (LoadError)
Bundle update datamapper
Bundle update couch_potato

Remove this load error:
cannot load such file -- json/add/rails (LoadError)
Bundle update dm-migrations datamapper dm-sqlite-adapter
Merge pull request #171 from sanemat/feature/update-datamapper

Update dm-migrations, datamapper, dm-sqlite-adapter, couch_potato for Ruby2.0.0-rc1