Spec started failing because a statement which we expected to be a
syntax error is now interpreted as a valid pattern-matching statement.
Swapping the hash-rockets for colons turns this back into a syntax
error.
CircleCI doesn't provide images for these Rubies since they are no onger
suported. I had to build them manually and push to dockerhub so we can continue
testing using those Rubies.
I am committing these Dockerfiles since they belong to this project and in case
we need to modify them.