Specify Ruby versions supported by Skunk

[bronzdoc]

Using ruby 1.8.7 😬

gem install skunk
Fetching: sexp_processor-4.14.1.gem (100%)
ERROR:  Error installing skunk:
        sexp_processor requires Ruby version ~> 2.2.

RubyGems Environment:
  - RUBYGEMS VERSION: 1.8.30
  - RUBY VERSION: 1.8.7 (2012-06-29 patchlevel 370) [x86_64-linux]
  - INSTALLATION DIRECTORY: /usr/local/lib/ruby/gems/1.8
  - RUBY EXECUTABLE: /usr/local/bin/ruby
  - EXECUTABLE DIRECTORY: /usr/local/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86_64-linux
  - GEM PATHS:
     - /usr/local/lib/ruby/gems/1.8
     - /root/.gem/ruby/1.8
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :benchmark => false
     - :backtrace => false
     - :bulk_threshold => 1000
  - REMOTE SOURCES:
     - http://rubygems.org/