Revert "Use ruby18 shim (w/ shebang) for remember file cmd"

This reverts commit 09fe0a35f1931cbfd098026308c4e4c58a59effc.

Commands using `RSpec::Mate::Runner` must not use the ruby18 shim (for now, at least).

Fixes “LoadError: cannot load such file -- bundler” when running this command in a project where a Gemfile exists.