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)
Merge pull request #7 from JoshRosen/travis

Add Travis CI configuration
Merge pull request #8 from shivaram/master

Move dev install directory to front of libPaths