fix(Gemfile): Make sure we install latest patch level of Rails

Without this we are not sure that the latest patch level gets installed
by bundler.

Author: tvdeyen

Gemfile

@@ -7,8 +7,8 @@ branch = ENV.fetch("SOLIDUS_BRANCH", "main")
 gem "solidus", github: "solidusio/solidus", branch: branch
 gem "solidus_backend", github: "solidusio/solidus", branch: branch
 
-rails_requirement_string = ENV.fetch("RAILS_VERSION", "~> 7.0")
-gem "rails", rails_requirement_string
+rails_requirement_string = ENV.fetch("RAILS_VERSION", "7.0")
+gem "rails", "~> #{rails_requirement_string}"
 
 # The solidus_frontend gem has been pulled out since v3.2
 if branch >= "v3.2"