Support for older version of ruby and rubygems

[maxprokopiev]

Hi guys! Have some spare time to give this project some love :heart:

Do we still need to support ruby 1.8 and older versions of rubygems? I think we should add

  s.required_ruby_version     = '>= 1.9.3'
  s.required_rubygems_version = '>= 1.8.11'

to our gemspec.

What do you think?

[maxprokopiev]

see more in https://github.com/caiges/Ruby-RC4/pull/8