Fixes check for user-specified definition

Better to use -f than -e to check for a user-specified definition file.
With -e, if the user accidentally types the name of a directory (or, far
less likely, a device file) with the same name as the Ruby they're
trying to install, they end up with ruby-build doing absolutely nothing
at best, or an error message that could be pretty confusing at worst.
Merge pull request #518 from dpoggi/better-user-specified-definition-check

Fixes #517