T: lock rails version to 2.3.12

- There are unfixed ActiveRecord regressions in 2.3.13
T: update gems
update travis config

- don't install development or production only gems
- DRY up env config
- disable email notifications
experiment with coveralls.io
add coveralls badge to readme
Merge pull request #1 from devfu/experiment/coveralls

Experiment with coveralls.io