Require Thor Dependency

Applications using StimulusReflex were failing
during deploys because of the Thor dependency
being referenced but not listed as a dependency.
This commit adds the require statement to the area
of the code that uses it but does not add it as a
dependency due to Thor being a dependency of
railties already.

lib/stimulus_reflex/installer.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "thor"
+
 module StimulusReflex
   class Installer
     include Thor::Base