Tentative ruby 3 fix

[hakanai]

Replaces the call to File.exists? with File.exist?.

Also updating the Rubies specified in .travis.yml to reflect the currently supported versions.

[hakanai]

Travis building not automated on this repo yet? I was hoping it might run and find me some more issues.

Otherwise I'll have to go and install all the different Ruby versions myself to check them. 💀

[zenspider]

last commit was 2018...