Fix for Less error during fab build

`Unable to load plugin clean-css please make sure that it is installed under or at the same level as less
Fatal error: Error running lessc on ...`

Downgraded Less:
    less/less-plugin-clean-css#23
    less/less.js#3181

package.json

@@ -3,7 +3,7 @@
  "version": "3.4.0",
  "description": "TimelineJS v3: A Storytelling Timeline built in JavaScript.",
  "dependencies": {
- "less": "^2.1.1",
+ "less": "2.7.3",
  "less-plugin-clean-css": "^1.2.0"
  },
  "devDependencies": {},