Restrict nokogiri to specific version ranges.
Restrict nokogiri to specific version ranges.

nokogiri 1.6.x is buggy => sparklemotion/nokogiri#1233

Until the bug is fixed, define a range of working nokogiri gems.

bramswenson/twemoji,vevix/twemoji,jollygoodcode/twemoji,AndyLampert/twemoji