Limit shoulda to shoulda-matchers

because contexts are already provided by RSpec

Gemfile

@@ -40,7 +40,7 @@ group :development, :test do
   gem 'factory_bot_rails'
   gem 'faker'
   gem 'rspec-rails'
-  gem 'shoulda'
+  gem 'shoulda-matchers'
 end
 
 group :development do

Gemfile.lock

@@ -171,10 +171,6 @@ GEM
       unicode-display_width (~> 1.4.0)
     ruby-progressbar (1.10.0)
     ruby_dep (1.5.0)
-    shoulda (3.6.0)
-      shoulda-context (~> 1.0, >= 1.0.1)
-      shoulda-matchers (~> 3.0)
-    shoulda-context (1.2.2)
     shoulda-matchers (3.1.2)
       activesupport (>= 4.0.0)
     solargraph (0.28.4)
@@ -231,7 +227,7 @@ DEPENDENCIES
   rails (~> 5.2.1)
   rspec-rails
   rubocop
-  shoulda
+  shoulda-matchers
   solargraph
   spring
   spring-watcher-listen (~> 2.0.0)