The minimum Ruby version required for byebug is now 2.2.0 thanks to deivid-rodriguez/byebug#337. This requirement change has led to our 2.1 builds failing with:

Gem::InstallError: byebug requires Ruby version >= 2.2.0.
Using github-linguist 5.2.0 from source at .
An error occurred while installing byebug (9.1.0), and Bundler cannot continue.
Make sure that `gem install byebug -v '9.1.0'` succeeds before bundling.
The command "eval bundle install --jobs=3 --retry=3 --path=${BUNDLE_PATH:-vendor/bundle} " failed 3 times.
The command "bundle install --jobs=3 --retry=3 --path=${BUNDLE_PATH:-vendor/bundle}" failed and exited with 5 during .
Your build has been stopped.

Rather than locking us to a particular version of byebug, I've bumped the minimum Ruby version to 2.2.0 and I've also defined a :debug group and excluded it in the Travis build so byebug is not installed during testing as it's not needed during testing.

One less gem means a slightly quicker build too 😄

/cc @github/data-engineering