gems: rename sha3 to keccak

[q9f]

replaces digest-sha3 to enable support for ruby 2.3 and later

• ref ext: rename sha3 to keccak for clarity q9f/keccak.rb#16

[a-mpch]

Hey @q9f, thanks for your contribution.
I don't get why digest-sha3 wouldn't support ruby >=2.3, could you elaborate more?

[q9f]

It nails the required ruby version at ~> 2.2 and is unmaintained since 2015. See phusion/digest-sha3-ruby#10

I'm maintaining the keccak gem as dependency of the eth gem. See se3000/ruby-eth#58

[q9f]

I also upgraded eth to 0.5 if you still maintain this, I can submit an additional PR.

• https://github.com/q9f/eth.rb/releases

[jgmontoya]

Updated it on #7