swap bash prework resource

Per @dbrizan's [suggestion](https://github.com/cuny-nytech/syllabus/pull/1#issuecomment-50953811).  Picked this one over [the Lynda one](http://www.lynda.com/Mac-OS-X-10-6-tutorials/Unix-for-Mac-OS-X-Users/78546-2.html), since I don't want to ask for the students to have to pay for access.

/cc https://github.com/cuny-nytech/syllabus/issues/7 @PeterBell