Updating cucumber / gherkin runner

Currently when using --group_by scenarios option gherkin runner generates command which outputs same feature files multiple times for instance:

cucumber features/feature1:10 features/feature1:14 features:feature2:13

This causes cucumber to call before_feature multiple times on the same feature within one thread. This is not desirable and a knock on effect is that junit reporter is broken as it overwrites feature files with the last execution.

This change fixes this by combining same features:

cucumber features/feature1:10:14 features:feature2:13
Incomplete commit previously
Only merging feature file arguments if group-by scenarios specified
Adding a space to trigger another build
Merge remote-tracking branch 'upstream/master'
Refactoring grouped_by sceanrios and adding a unit test
Correcting cucumber hook behaviour: Needed to set group_by option to trigger new grouping
Merge pull request #325 from sponte/master

Updating cucumber / gherkin runner