npm 2.x and node v4 on Travis

Use latest v2 release of npm as the just released v3 doesnt seem to be node v0.8 compliant atm.

And test on node v4 for obvious reasons.

.travis.yml

@@ -3,8 +3,9 @@ node_js:
   - 0.8
   - '0.10'
   - 0.12
+  - 4
 before_install:
-  - npm install -g npm
+  - npm install -g npm@2.x
 script:
   - npm test
   - "test $TRAVIS_PULL_REQUEST != 'false' || test $TRAVIS_NODE_VERSION != '0.12' || npm run test-browser"