Tweak actions

This gem exposes no executables.

YAML.

gemspec: Explicitly empty executables list

CI: Quote 3.0 in YAML to avoid a String "3"

This avoids shelling out to git.

gemspec: Explicit file list

Improves the method's performance when asked to remove padding.

str.delete!("=") iterates over the entire string looking for the equals
character, but we know that we will, at most, find two at the end of the
string.

Thanks @nobu!

…ormance-removing-padding

Improve Base64.urlsafe_encode64 performance

Maybe some long time ago this used minitest? That's not the case anymore.

String#ljust returns a new string, so whenever we need to add
padding, we can replace "-/" in place with String#tr! and avoid creating
yet another copy of the string.

Avoid unnecessary string duplication in .urlsafe_decode64