fix(travis-ci): Speed up the build for TravisCI

This commit speeds up the builds by upto 3 times than the current.
It uses npm 3 over npm 2 (travis default), and caches the
dependencies.

This is okay, because our prime two testing areas are, linting and
tests for the challenges.

NOTE: The speed up should happen from the second build,
as it will take atleast one build to cache the dependencies.

.travis.yml

@@ -3,6 +3,12 @@ language: node_js
 node_js:
   - '4.2.1'
 
+before_install: if [[ `npm -v` != 3* ]]; then npm i -g npm@3; fi
+
+cache:
+  directories:
+    - node_modules
+
 env:
   - CXX=g++-4.8