[Node 4] Update .travis.yml to install Node 4.x + upgrade jest

Jest 0.5.2 ships with a fix for bound functions, which RN exercises. And with this Travis change, the automated tests will install Node 4.x with nvm before running.

Test Plan:`nvm use 4 && npm i && npm test`
See if Travis tests pass.

.travis.yml

@@ -15,7 +15,7 @@ install:
   - mkdir -p .nvm
   - export NVM_DIR="$PWD/.nvm"
   - source $(brew --prefix nvm)/nvm.sh
-  - nvm install iojs-v3
+  - nvm install 4
   - rm -Rf `node -p "require('os').tmpDir()"`/jest_preprocess_cache
   - npm install -g flow-bin@`node -p "require('fs').readFileSync('.flowconfig', 'utf8').split('[version]')[1].trim()"`
   - npm config set spin=false