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
Something more like:
```ruby
  (testpath/".java-version").write "homebrew-test"
  output = `jenv version`
  assert output.include? `version homebrew-test is not installed`
```