Update Spark requirements
Update TODO.md
Update README to use install_github
Merge pull request #4 from shivaram/master

Update README to use install_github
Use one style ($) for accessing names in environments.
Merge pull request #6 from concretevitamin/env-style-fix

Use one style ($) for accessing names in environments.
Add Travis CI integration (using craigcitro/r-travis)