Migrate from Travis to Github Actions

[n-rodriguez]

I intend to create a PR with this :

• migrate from Travis to Github Actions
• moving lib specs under spec/closure_tree/ (instead of specs)
• disable RSpec monkey patching

Will it be merged?

@seuros I know you want to move away from RSpec (#316) but it could be a first step towards it : having clean specs to ease the rewrite to another framework.

[n-rodriguez]

@seuros Maybe we could use https://github.com/seattlerb/minitest#specs- to ease the migration? wdyt?