Fix to always use the Java version specified in JAVA_HOME

Here's a nice one... Puts whatever in JAVA_HOME first on the PATH and therefore we can continue to use 'java' to get the correct version. Not the prettiest solution out there but having tried a couple of other approaches this is the first one I have actually made work the way we want it to. 

It's been tested on Windows 8.

Special review by @jroper (whom I know is a huge fan of Windows batch scripts).

It should be local because we issue these commands:
https://github.com/typesafehub/activator/blob/master/dist/src/templates/activator.bat#L8
https://github.com/typesafehub/activator/blob/master/dist/src/templates/activator.bat#L222