Make tmp directory if it doesn't exist

In mina-deploy/mina#225, gabskoro noted that the tmp folder might not
exist in all Rails projects. So, touching restart.txt won't actually do
anything.

This creates the tmp directory, in case it doesn't exist, with mkdir -p.
Unfortunately, there isn't a flag for touch which forces intermediate
directory creation, so we've got to use a separate command.
Merge pull request #229 from dvdhsu/master

Make tmp directory if it doesn't exist