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SCSS-Lint 0.49.0 modifies Shorthand linter to report lint if a shorthand
of a length not specified in the allowed_shorthands option is used.

New defaults include "4" in the list of allowed shorthands, so we are changing our configuration accordingly.

Ref: https://github.com/brigade/scss-lint/commit/e283d1689699f581561fea344df3168128c46d7b

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Jekyll 3.x has it as a dependency already.

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Add jekyll-redirect-from plugin in Gemfile, ohterwise report ".../jekyll/plugin_mannager.rb:29:in require: cannot load such file -- jekyll-redirect-from (LoadError)" when jekyll build

* Separate configs for libsass and sass.
* Sass compiler selected based on `process.env.TWBS_SASS`.
* Travis:
  * Use Gemfile to manage ruby dependencies.
  * Run core tests with both Sass compilers.
  * Only install/cache ruby gems required by the test subset.
* Grunt: `update-gemfile-lock` task a la `update-shrinkwrap`.