Pin node-sass dependency to v3.4.x

Fix build by pinning node-sass dependency (used indirectly via
gulp-sass) to v3.4.x until
https://github.com/Igosuki/compass-mixins/pull/86 is resolved.

package.json

@@ -49,6 +49,7 @@
     "lodash.debounce": "^4.0.3",
     "mkdirp": "^0.5.1",
     "ng-tags-input": "2.2.0",
+    "node-sass": "~3.4.1",
     "node-uuid": "^1.4.3",
     "postcss": "^5.0.6",
     "postcss-url": "^5.1.1",