For the sake of cleanliness.

v6 is the current stable version and will be an LTS version.
v5 is not an LTS version.

Refs https://github.com/nodejs/LTS

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This reverts commit d0059ac2.

Node.js 5.7.1 is out with the issue fixed.

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This should be reverted when the next 5.x version is out.

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See https://github.com/npm/npm/issues/11283

See https://github.com/travis-ci/travis-ci/issues/5092 for more details.
Reverts #18376

So that the build will stop failing due to https://github.com/travis-ci/travis-ci/issues/5092

"stable" is deprecated by nvm
Refs https://github.com/twbs/grunt-bootlint/pull/61
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Fixes #18225.

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Reverts part of 42697a4e

Reverts a1c170ed

Instead, use `bundleExec: true` for grunt-scss-lint.

[ci skip]

[skip sauce]

* 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`.