Compile with Ruby Sass or libsass.

* 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`.

grunt/bs-sass-compile/libsass.js

@@ -0,0 +1,26 @@
+// Compile Bootstrap with [libsass][1] using [grunt-sass][2]
+// [1]: https://github.com/sass/libsass
+// [2]: https://github.com/sindresorhus/grunt-sass
+module.exports = function configureLibsass(grunt) {
+  grunt.config.merge({
+    sass: {
+      options: {
+        includePaths: ['scss'],
+        precision: 6,
+        sourceComments: false,
+        sourceMap: true
+      },
+      core: {
+        files: {
+          'dist/css/<%= pkg.name %>.css': 'scss/<%= pkg.name %>.scss'
+        }
+      },
+      docs: {
+        files: {
+          'docs/assets/css/docs.min.css': 'docs/assets/scss/docs.scss'
+        }
+      }
+    }
+  });
+  grunt.loadNpmTasks('grunt-sass');
+};

grunt/bs-sass-compile/sass.js

@@ -0,0 +1,30 @@
+// Compile Bootstrap with [Ruby Sass][1] using [grunt-contrib-sass][2]
+// [1]: https://github.com/sass/sass
+// [2]: https://github.com/gruntjs/grunt-contrib-sass
+module.exports = function configureRubySass(grunt) {
+  var options = {
+    loadPath: ['scss'],
+    precision: 6,
+    sourcemap: 'auto',
+    style: 'expanded',
+    trace: true,
+    bundleExec: true
+  };
+  grunt.config.merge({
+    sass: {
+      core: {
+        options: options,
+        files: {
+          'dist/css/<%= pkg.name %>.css': 'scss/<%= pkg.name %>.scss'
+        }
+      },
+      docs: {
+        options: options,
+        files: {
+          'docs/assets/css/docs.min.css': 'docs/assets/scss/docs.scss'
+        }
+      }
+    }
+  });
+  grunt.loadNpmTasks('grunt-contrib-sass');
+};