Remove redundant encoding and require_relative check

[tagliala]

This magic comment is not needed anymore because the default encoding of source code files assumed by Ruby 2 is utf-8, and the minimum required Ruby version is 2.6

Auto-fixed via:

rubocop -a --only Style/Encoding

---

require_relative has been introduced in Ruby 1.9.2 and is available in all supported Ruby versions

[tagliala]

Failures are still unrelated

Pending: (Failures listed here are expected and do not affect your suite's status)
/Users/runner/work/ethon/ethon/vendor/bundle/ruby/2.7.0/gems/ffi-1.17.2-arm64-darwin/lib/ffi/autopointer.rb:152: [BUG] Segmentation fault at 0x0000000000000020

As I mentioned in #241 , I've experienced several segmentation faults back at the time

[felipedmesquita]

Thanks, @tagliala!