ruby: remove lz4 and lz4hc support

see @tsee's comment at 050f349a17ae80af8f29db1801797f4f569136fe
i am not leaving any trace of it since it was released only a week
ago and only bots have downloaded the gem from rubygems.org.

and i also dont know any graceful way of handling my mistake
of pushing the lz4 support into the master branch.

so if someone is using LZ4 you can downgrade to 0.0.5 by using:
    gem 'sereal', '= 0.0.5'
or
    $ gem install sereal -v 0.0.5