This change updates most libraries used in our Grunt build stack and
applies necessary changes to Grunt and Gherkin-lint configuration to
ensure that they continue to work.

The grunt-sass plugin has been updated to support alternative
'implementations' of sass compilers, and the chosen sass compiler must
now be specified in the grunt configuration. We continue to use the
`node-sass` package for this.

Our gherkin-lintrc included two rules which were renamed from
'no-unamed-*' to 'no-unnamed-*'. This change occurred in version 2.0.0
of Gherkin-lint and has no other effect.