Don't use spring in the test environment

Spring now (since version 3, I think) requires `config.cache_classes =
false` in order to work. However, that means that tests would be slower
for developers who don't use spring (like me).

I'd personally vote for removing spring completely (Rails removed it as
a default installation option in August 2021 [1]), but for now we're
keeping it for backwards compatibility.

[1] See pull request 42997 in https://github.com/rails/rails

Gemfile

@@ -77,8 +77,6 @@ group :development, :test do
   gem "knapsack_pro", "~> 5.7.0"
   gem "launchy", "~> 2.5.2"
   gem "letter_opener_web", "~> 2.0.0"
-  gem "spring", "~> 4.1.3"
-  gem "spring-commands-rspec", "~> 1.0.4"
 end
 
 group :test do
@@ -114,6 +112,7 @@ group :development do
   gem "rubocop-rspec", "~> 2.27.0", require: false
   gem "rvm1-capistrano3", "~> 1.4.0", require: false
   gem "scss_lint", "~> 0.60.0", require: false
+  gem "spring", "~> 4.1.3"
   gem "web-console", "~> 4.2.1"
 end