Ability to specify PhantomJS version

[cloakedninjas]

We needed to get v2.0.0 of Phantom in our app but having 1.9 hard coded was causing issues, so for anyone else who might want it, here is a way to get it

[nicks]

hmmm...this is a cool trick, but I think it will just cause more problems:

1. it breaks the hermetic-ness of package.json (i.e., what's in package.json doesn't match the code installed)
2. it breaks npm's assumptions about shared deps (i.e., package A and package B say they both depend on phantomjs 1.9.8, so npm lets them share dependencies, but package B actually requires 2.0.0 via a special env variable)
3. it actually won't work on most platforms because of the upstream phantomjs bugs (see 2.0 #288)
   Sorry, I wish there was a better solution, but I think we just need to fix phantomjs upstream.