Added R install on OS X using a binary distribution from CRAN

Craig,

I got r-travis to build on OS X in travis-ci.  See the build on travis: https://travis-ci.org/petrifast/r-travis/builds/13102756

Please take a look at the changes mainly in scripts/travis-tools.sh within BootstrapMac().

Instead of using homebrew to build R, I simply download a binary distribution (R-latest.pkg) from CRAN and install it. This bypasses a long build and avoids the issue of the missing X11 server. This works nicely and is much faster than installing from homebrew.

The build on OS X is triggered by choosing language: objective-c. One has to also comment out the line with 'tmux' in .travis.yml in order to build on OS X. See the comments in .travis.yml.  

The version of .travis.yml in my fork defaults to a Linux build that is ok, see https://travis-ci.org/petrifast/r-travis

Once travis-ci allows looping over {Linux, OS X}, I'll be happy to help get both builds working. If you have any thoughts or questions let me know.

Petri