Update to test node stable and remove pre-install

The node version installed nowadays on Travis is good enough, so no need
to install it before running the tests.

.travis.yml

@@ -3,9 +3,7 @@ sudo: false
 node_js:
 - "0.12"
 - "0.10"
-- iojs
-before_install:
-- npm install -g npm@'>2.7.0'
+- "4"
 after_script:
 - CODECLIMATE_REPO_TOKEN=7b0cbee1382c968a036868d26ec04d0ddc7b7aeef25ceead5ff9ff50a3c2ae8b codeclimate < coverage/lcov.info
 - rm -rf ./coverage