Umlaut app generator steps fail if dependencies not already installed

[ryanfb]

When I run e.g. umlaut my_new_app, during the generation process I see the lines:

       apply  /Users/ryan/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/umlaut-4.1.7/lib/generators/umlaut_app_template.rb
      append    Gemfile
    generate    umlaut:remove_turbolinks
Could not find gem 'mysql2 (< 0.5, >= 0.3.13)' in any of the gem sources listed in your Gemfile or available on this machine.
Run `bundle install` to install missing gems.
    generate    umlaut:install
Could not find gem 'mysql2 (< 0.5, >= 0.3.13)' in any of the gem sources listed in your Gemfile or available on this machine.
Run `bundle install` to install missing gems.
         run  bundle install

At the end of this process, there's no ./config/umlaut_services.yml file in the app directory (presumably generate umlaut:install failed due to missing dependencies). If I run bundle exec rails generate umlaut:install again afterwards, the file gets generated because the Rails generator ran bundle install after https://github.com/team-umlaut/umlaut/blob/master/lib/generators/umlaut_app_template.rb.

There's probably a few different ways to go about fixing this:

1. Add a command that runs bundle install to https://github.com/team-umlaut/umlaut/blob/master/lib/generators/umlaut_app_template.rb after appending to the Gemfile, but before calling generate. This may be a little redundant because apparently the default Rails generator will run bundle install afterwards anyway.
2. Add mysql2 to the umlaut.gemspec dependencies, so that after a user runs gem install umlaut, they'll already have the mysql2 dependency when they run umlaut my_new_app (which passes -d mysql to the Rails generator).
3. Move the generate calls in umlaut_app_template.rb into an after_bundle block. This is mentioned there as available in future versions of Rails, and looks like it was added in 4.2.x, so this would be a clean solution but break backward compatibility with other versions of Rails.

[jrochkind]

Hmm, thanks for catching this, annoying. #3 sounds reasonable to me, if it works -- I think it's probably okay to require Rails 4.2 at this point, especially for future releases. We could also put some conditional logic in that does #3 if Rails.version is at least 4.2, but does it the old somewhat broken way in earlier Rails?

Interested in submitting a PR?