[FIX] Prevent pre-2.1 rubies from installing nokogiri 1.7+
[FIX] Prevent pre-2.1 rubies from installing nokogiri 1.7+

Nokogiri 1.7.0 will not install on ruby < 2.1.0

https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md#170--2016-12-26

benlangfeld/ruby_speech,benlangfeld/ruby_speech,adhearsion/ruby_speech,adhearsion/ruby_speech,adhearsion/ruby_speech,benlangfeld/ruby_speech