Gvm update and cask plugin

simple added plugin that just puts cask on the path.

Fixed gvm plugin so that the HOME variables are set for all installed gvm binaries, also set JAVA_HOME the way gvm would do it for OSX if JAVA_HOME is not currently set.

Let me know if there is anything else you need for this :)
if these paths were just regular files it wouldn't make sense to set `JAVA_HOME` here either.  The directory check is just to help ensure that the `bin` folder resides under one of these `java_homes`