Avoid JRuby warning during rehash Rubygems plugin

As it seems, JRuby 1.7 complains on stderr every time you invoke `system`:

    warning: executable? does not in this environment and will return a dummy value

It doesn't seem to complain when backtics are used. It's safe to use
backticks here because `rbenv rehash` doesn't output anything on stdout,
and the exit status of the command is irrelevant.