Fix gherkin-lint issues and enable on travis

[jhedstrom]

In #504 gherkin-lint was added to package.json but not enabled. We currently have these fails:

> gherkin-lint features

/Users/jhedstrom/work/contributions/utilities/drupalextension/features/api_background.feature
  1      Wrong indentation for "feature tag", expected indentation level of 0, but got 12     indentation


/Users/jhedstrom/work/contributions/utilities/drupalextension/features/api.feature
  9      Wrong indentation for "scenario tag", expected indentation level of 2, but got 17    indentation
  15     Wrong indentation for "scenario tag", expected indentation level of 2, but got 13    indentation
  23     Wrong indentation for "scenario tag", expected indentation level of 2, but got 13    indentation
  29     Wrong indentation for "scenario tag", expected indentation level of 2, but got 17    indentation
  35     Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  41     Wrong indentation for "scenario tag", expected indentation level of 2, but got 17    indentation
  48     Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  65     Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  75     Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  83     Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  103    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  111    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  161    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  176    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  195    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  203    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  211    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  216    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  247    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation


/Users/jhedstrom/work/contributions/utilities/drupalextension/features/config.feature
  1      Wrong indentation for "feature tag", expected indentation level of 0, but got 5      indentation


/Users/jhedstrom/work/contributions/utilities/drupalextension/features/d6.feature
  1      Wrong indentation for "feature tag", expected indentation level of 0, but got 5      indentation


/Users/jhedstrom/work/contributions/utilities/drupalextension/features/d8.feature
  1      Wrong indentation for "feature tag", expected indentation level of 0, but got 4      indentation


/Users/jhedstrom/work/contributions/utilities/drupalextension/features/drush.feature
  1      Wrong indentation for "feature tag", expected indentation level of 0, but got 11     indentation


/Users/jhedstrom/work/contributions/utilities/drupalextension/features/field_handlers.feature
  9      Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  43     Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  70     Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  83     Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  96     Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  113    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  144    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation
  157    Wrong indentation for "scenario tag", expected indentation level of 2, but got 6     indentation


/Users/jhedstrom/work/contributions/utilities/drupalextension/features/language.feature
  1      Wrong indentation for "feature tag", expected indentation level of 0, but got 9      indentation


/Users/jhedstrom/work/contributions/utilities/drupalextension/features/mail.feature
  1      Wrong indentation for "feature tag", expected indentation level of 0, but got 5      indentation


/Users/jhedstrom/work/contributions/utilities/drupalextension/features/messages.feature
  1      Wrong indentation for "feature tag", expected indentation level of 0, but got 5      indentation


/Users/jhedstrom/work/contributions/utilities/drupalextension/features/random.feature
  1      Wrong indentation for "feature tag", expected indentation level of 0, but got 9      indentation