Remove `gem install scss_lint` from Travis.

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

.travis.yml

@@ -13,7 +13,6 @@ before_install:
   - if [ "$TRAVIS_REPO_SLUG" = twbs-savage/bootstrap ]; then export TWBS_DO_VALIDATOR=0; fi
 install:
   - bundle install --deployment --jobs=3
-  - gem install scss_lint --no-rdoc --no-ri
   - npm install -g grunt-cli
   - npm install
 cache:

Gruntfile.js

@@ -206,6 +206,7 @@ module.exports = function (grunt) {
     // CSS build configuration
     scsslint: {
       options: {
+        bundleExec: true,
         config: 'scss/.scsslint.yml',
         reporterOutput: null
       },