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*
is this better ?
```Ruby
grouped = test_files.map { |t| t.split(":") }.group_by(&:first)
combined_scenarios = grouped.map {|file,files_and_lines| files_and_lines.map(&:last).join(":") }
```