Make rbenv:validate task give positive output

Fixes #23. Now nobody worries since we don't output 'failed'.

    DEBUG [c7501a58] Running /usr/bin/env [ -d /opt/rbenv/versions/1.9.3-p484 ] on yourserver.com
    DEBUG [c7501a58] Command: [ -d /opt/rbenv/versions/1.9.3-p484 ]
    DEBUG [c7501a58] Finished in 0.511 seconds with exit status 0 (successful).