fix prompts when using mockPrompts

don't want to upgrade `yeoman-generator` dependency and start breaking other
random stuff just yet. `"yeoman-generator": "~0.13.4"` vs. 0.18.x
typo? js -> jsDir
fix Gemfile template for testing purposes
override GEM_HOME for bundle exec during tests
fix test-creation.js w.r.t jekyllrb:app