Make rails5_ci_fails_no_database tests use Heroku-18 [skip changelog] (#174)

The `rails5_ci_fails_no_database` test fixture now fails after the
Heroku default stack changed from `heroku-18` to `heroku-20`,
since the fixture's `Gemfile.lock` specifies Ruby 2.4.1, when the
oldest Ruby on Heroku-20 is Ruby 2.5.8.

As a stop-gap until the external-repo fixture is updated to use
newer Ruby, the tests now explicitly use the Heroku-18 stack
rather than whatever happens to be the platform default.

Fixes #173.

Author: edmorley

Diff for: spec/hatchet/ci_spec.rb

@@ -28,7 +28,7 @@
 
   it "error with bad app" do
     expect {
-      Hatchet::GitApp.new("rails5_ci_fails_no_database").run_ci { }
+      Hatchet::GitApp.new("rails5_ci_fails_no_database", stack: "heroku-18").run_ci { }
     }.to raise_error(/PG::ConnectionBad: could not connect to server/)
   end
 
@@ -41,7 +41,7 @@
       @before_deploy_dir_pwd = Dir.pwd
     end
 
-    Hatchet::GitApp.new("rails5_ci_fails_no_database", allow_failure: true, before_deploy: before_deploy).run_ci do |test_run|
+    Hatchet::GitApp.new("rails5_ci_fails_no_database", stack: "heroku-18", allow_failure: true, before_deploy: before_deploy).run_ci do |test_run|
       expect(test_run.status).to eq(:errored)
       expect(@before_deploy_dir_pwd).to eq(Dir.pwd)
       expect(@before_deploy_called).to be_truthy