[v0] Group ruby tests

Group execution of the rspec specs under the ruby test, and use gemnasium:2.3.0 as a base image. This ensures that the version of Ruby used when running the tests matches the one used when running the scanner.

When changing the Ruby version it appeared that there were flaky tests, and this MR also fixes that.

Finally, the MR adds empty lines in the spec, making them easier to read.

This is an addition to !47 (merged)