Depend on solidus_frontend in GH for development

In development, we don't want Bundler to use solidus_frontend
from RubyGems. That's because we don't always have a development
version released for any version of Solidus.

This will allow us to always use the last development code,
without the need of releasing a new .dev version on RubyGems.

Gemfile

@@ -56,6 +56,7 @@ group :backend, :frontend do
 end
 
 group :frontend do
+  gem 'solidus_frontend', github: 'solidusio/solidus_frontend', branch: 'v3.2'
   gem 'generator_spec'
 end