Switch from travis to github actions for unit tests (#1206)

Migrate to github actions as travis has switched to a process that will
require requesting minutes regularly for open source projects, and
current development is slow enough that this additional overhead is too
much.

Correct the generation of the line coverage following the example at
coverallsapp/github-action#29. Remove dependency on coveralls
gem as no longer needed if using the action.

Patch older versions of simplecov to contain a branch_coverage?
method to ensure working with simplecov-lcov.

Gemfile

@@ -36,5 +36,3 @@ end
 group :plugins do
   gemspec
 end
-
-gem 'coveralls', require: false