Test only one patch version, add Ruby 2.7 to CI

[aried3r]

Unless there's a reason to test certain patch versions, this change will make Travis pick the latest available patch version. It will also reduce test times.

cache: bundler should also speed up future CI runs.

script: was removed, since the default task runs RSpec and rake is being run by Travis

reverse_markdown/Rakefile

Lines 8 to 9 in e2c6973

	RSpec::Core::RakeTask.new(:spec)
	task :default => :spec

[xijo]

Fair enough! 👍