Fix puppet drush install class command

Upon: vagrant provision, the following error occurred:

...
Notice: /Stage[main]/Drush/Exec[drush install]/returns: ERROR: can't find php.
Error: rm -rf * && curl -L https://api.github.com/repos/drush-ops/drush/tarball/6.0.0 | tar --strip-components 1 -xzf - && ./drush returned 1 instead of one of [0]
Error: /Stage[main]/Drush/Exec[drush install]/returns: change from notrun to 0 failed: rm -rf * && curl -L https://api.github.com/repos/drush-ops/drush/tarball/6.0.0 | tar --strip-components 1 -xzf - && ./drush returned 1 instead of one of [0]
Notice: /Stage[main]/Drush/File[/usr/local/bin/drush]: Dependency Exec[drush install] has failures: true
Warning: /Stage[main]/Drush/File[/usr/local/bin/drush]: Skipping because of failed dependencies
Notice: /Stage[main]/Base/Package[ImageMagick]/ensure: created
...

This patch seems to fix the issue.