jenv 0.4.0, a rbenv-like for Java

Initial formula for jenv 0.4.0

Thanks

see : https://github.com/Homebrew/homebrew/pull/17406#issuecomment-60201953
Can a test be added to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. Thanks!