Generate Gemfile entry with version constraint

When generating a new application by loading the template over HTTP,
specifying `gem "suspenders"` without a version constraint results in
the resolution of an extremely outdated version ([v0.2.4][]).

When omitting the `--suspenders-main` argument, include a version
constraint to use a version that is at least `3.0.0`.

[v0.2.4]: https://github.com/thoughtbot/suspenders/tree/v0.2.4

lib/install/web.rb

@@ -28,7 +28,7 @@ def apply_template!
         if ARGV.include?("--suspenders-main")
           gem "suspenders", github: "thoughtbot/suspenders", branch: "main"
         else
-          gem "suspenders"
+          gem "suspenders", "~> 3"
         end
       end
 

test/generators/suspenders/install/web_generator_test.rb

@@ -12,6 +12,14 @@ class WebGeneratorTest < Rails::Generators::TestCase
         setup :prepare_destination
         teardown :restore_destination
 
+        test "generates Gemfile entry with version constraint" do
+          with_database "postgresql" do
+            run_generator
+          end
+
+          assert_file "Gemfile", /gem "suspenders", "~> 3"/
+        end
+
         test "raises if API only application" do
           within_api_only_app do
             assert_raises Suspenders::Generators::APIAppUnsupported::Error do