Add a DB env var for testing against postgres, mysql
Drop quoted columns in SET clauses. Fixes #64, Reverts #60.

There is no One Quoting Syntax To Rule Them All. For a SET statement,
postgres and mysql seems to disagree about how a column name should be
quoted if it is quoted at all. Here, we go back to un-quoted column
names. Though columns named "order" and other reserved words will cause
errors, the solution works in all other cases.
Test MySQL and Postgres on Travis
Bump the version of Rails 3.2 in Travis tests