Remove test task; pliny apps use RSpec task now

- `rake -T` in pliny-template lists a 'rake test' task but it does not
  do anything with current pliny-template since tests have been moved to
  RSpec + Minitest.
Remove mention of rake test

- Note that the default rake RSpec task is provided by pliny-template.
- This will likely stop being a concern when #83 is merged and the
  documentation for pliny-template is split out of this README.
Merge pull request #86 from mathias/remove-test-task

Remove test task; pliny apps use RSpec task now