Specify which bundler version to install

We're now using the same version we used to generate our Gemfile.lock.

Using the latest bundler we got a deprecation warning, which might turn
into an error in the future:

[DEPRECATED] The `--deployment` flag is deprecated because it relies on
being remembered across bundler invocations, which bundler will no
longer do in future versions.

We could also upgrade to bundler 2.x, but since we're using Ruby 2.4 and
Ruby 2.6 comes with bundler 1.17, we've decided to keep this version.

config/deploy.rb

@@ -71,7 +71,7 @@ end
 task :install_bundler_gem do
   on roles(:app) do
     within release_path do
-      execute :rvm, fetch(:rvm1_ruby_version), "do", "gem install bundler"
+      execute :rvm, fetch(:rvm1_ruby_version), "do", "gem install bundler --version 1.17.1"
     end
   end
 end