Make mini_portile2 a development dependency, as it is only needed
for building.

Index: .metadata
--- .metadata.orig
+++ .metadata
@@ -19,7 +19,7 @@ dependencies:
     - - "~>"
       - !ruby/object:Gem::Version
         version: 2.8.0
-  type: :runtime
+  type: :development
   prerelease: false
   version_requirements: !ruby/object:Gem::Requirement
     requirements: