Updated README.md

Recommend either Homebrew or MacPorts for Mac OSX:
* Add MacPorts command for installing GNU Parallel.
* Add Homebrew commands for Mac OSX Ruby install workaround.

Fixed typo.
Merge pull request #21 from bradydoll/readme-macports-homebrew

Update README to recommend either Homebrew or MacPorts for Mac OSX
Release 0.6.1
Use Java 7 by default