Feature Request: Test Basic Gem Installation on Images

[ksylvest]

Describe the Feature Request
Currently it appears no tests run on any images to ensure gems install properly / ruby is configured. Suggest having a basic test run to install a list of common gems ensuring ruby is properly installed gem install bcrypt / gem install rails / gem install pg (fairly common gems).

Is your feature request related to a particular problem?
The 3.3 images were published without jemalloc installed causing issues in installing all gems (see: #156).

How will this feature request benefit CircleCI jobs using this image?
This'll ensure the images work when published.

Describe the solution you would like to see
Run the tests on building the images.

Describe alternatives you have considered
N/A