Remove unnecessary options for feature test task

For example, we do not need to generate a results.html, these tests are run on the command line.
Remove unnecessary tasks and dependencies
Organise tasks for clarity
Change features to feature

To match other test tasks.
Remove missing things from gitignore

Probably could remove more of these but this is just a quick tidy up.
Change feature back to features

Seems this is standard in Cucumber https://github.com/cucumber/cucumber/wiki/Running-Features so changing it back here.

Also adding a line to the README to say how to run the tests individually.
Merge pull request #6 from alphagov/tidy-dependencies

Tidy dependencies