More effective install-time workaround for Java

Use `PlistBuddy` and some `ln` jiggery-pokery to make JDK 7 the bulletproof default JDK on supported versions of OS X. Not dead-set on this getting merged, but it certainly makes my life easier - following installation of this modified Cask, subsequent installation of `android-studio`, `appcode`, `eclipse-ire` are tested to work immediately with no Info.plist `sed`-ing.