Fix CI on Solidus v3.1 by explicitly fetching solidus_frontend from GH

This is the same fix that was used in
9f1f60b. Otherwise, bundler will
try to fetch the gem from RubyGems and complain because not finding
`v3.1.10.dev` version.

Closes #239

Author: waiting-for-dev

Gemfile

@@ -8,8 +8,13 @@ gem 'solidus', github: 'solidusio/solidus', branch: branch
 gem 'solidus_backend', github: 'solidusio/solidus', branch: branch
 
 # The solidus_frontend gem has been pulled out since v3.2
-gem 'solidus_frontend', github: 'solidusio/solidus_frontend' if branch == 'master'
-gem 'solidus_frontend' if branch >= 'v3.2' # rubocop:disable Bundler/DuplicatedGem
+if branch >= 'v3.2'
+  gem 'solidus_frontend'
+elsif branch == 'master'
+  gem 'solidus_frontend', github: 'solidusio/solidus_frontend'
+else
+  gem 'solidus_frontend', github: 'solidusio/solidus', branch: branch
+end
 
 case ENV.fetch('DB', nil)
 when 'mysql'