Add .byebug_history to generated plugin .gitignore

The generated plugin Gemfile includes the Byebug gem (commented out), so
it makes sense to include the history file in the generated .gitignore.

railties/lib/rails/generators/rails/plugin/templates/gitignore.tt

@@ -17,3 +17,4 @@ pkg/
 <% end -%>
 <%= dummy_path %>/tmp/
 <% end -%>
+.byebug_history