Renamed gardens.rb to gardens_spec.rb so that these tests will run.

As discussed in http://talk.growstuff.org/t/test-coverage-mystery/87 , the tests in spec/features/gardens.rb were not being run.  Changing the filename to include _spec makes the testing framework automatically pick them up and increases the test coverage percentage.

Looks like this could link to story 78615584 Increase test coverage to > 90% (the 'copy link' isn't working for me in Safari) in Pivotal and check off the "write basic CRUD feature tests for gardens" task.