[Javascript] Limit Built Branches on Travis

By default, Travis will build all branches, tags, and Pull Requests. Because we're building our master branch before we release, we don't need Travis building our releases. Also, we don't care to have Travis build all the branches. So we're going to limit Travis to only build our master branch and Pull Requests by configuring travis via our .travis.yml file.

sudo: false
language: node_js
cache:
  directories:
    - node_modules
branches:
  only:
    - master
notifications:
  email: false
node_js:
  - iojs
before_install:
  - npm i -g npm@^2.0.0
before_script:
  - npm prune
script:
  - npm run test:single
  - npm run check-coverage
  - npm run build
after_success:
  - npm run report-coverage
  - npm run semantic-release