Use specific rake version to fix Gem::InstallError

12.2.1 is the last version which supports ruby 1.9.

```
  1) Error:
AcceptanceTest#test_binstub_upgrade_with_old_binstub:
RuntimeError: command failed
$ bundle check || bundle update --retry=2
--- stdout ---
Bundler can't satisfy your Gemfile's dependencies.
Install missing gems with `bundle install`.
Fetching gem metadata from http://rubygems.org/...............
Fetching gem metadata from http://rubygems.org/..
Resolving dependencies....
Fetching rake 12.3.1
Installing rake 12.3.1
Gem::InstallError: rake requires Ruby version >= 2.0.0.
An error occurred while installing rake (12.3.1), and Bundler cannot continue.
Make sure that `gem install rake -v '12.3.1' --source 'http://rubygems.org/'`
succeeds before bundling.
```

https://travis-ci.org/mtsmfm/spring-debug/jobs/404541780

Author: mtsmfm

lib/spring/test/application_generator.rb

@@ -97,6 +97,10 @@ def install_spring
           append_to_file(application.gemfile, "gem 'nokogiri', '~> 1.6.8'")
         end
 
+        if RUBY_VERSION < "1.9"
+          append_to_file(application.gemfile, "gem 'rake', '12.2.1'")
+        end
+
         application.bundle
 
         FileUtils.rm_rf application.path("bin")