rbenv.rake: using a simpler File.directory? test

This is preferable to forking, and the previous test did not seem to work.

Error seen:
```
DEBUG [a4479203] Running /usr/bin/env [ ! -d /usr/local/rbenv/versions/jruby-1.7.11; ] on vagrant-devtest
DEBUG [a4479203] Command: [ ! -d /usr/local/rbenv/versions/jruby-1.7.11; ]
DEBUG [a4479203] 	bash: line 0: [: missing `]'
DEBUG [a4479203] 	bash: ]: command not found
```