Make mysql the default for all dev and test

Managing multiple databases is a nightmare. It's not too bad
to set up MySQL on each machine.
Remove default value from comments

Blob/Text columns can't have default values in MySQL
Make spring and RSpec play nice together

See https://github.com/jonleighton/spring/issues/21
Fix feature specs by removing transactional fixtures

We'll use Database cleaner instead, because apparently
rspec's `config.use_transactional_fixtures` causes a database lock.
Remove default comment body from original migration