Restrict nokogiri to specific versions
Restrict nokogiri to specific versions

- As identified in https://github.com/jch/html-pipeline/pull/170,
  nokogiri 1.6.x is buggy
  - Team nokogiri is working on the fix
    https://github.com/sparklemotion/nokogiri/issues/1233
- Until the bug is fixed, define a range of working nokogiri gems

jch/html-pipeline,gjtorikian/html-pipeline,terracatta/html-pipeline,kbrock/html-pipeline,jch/html-pipeline,github/html-pipeline,terracatta/html-pipeline,github/html-pipeline,kbrock/html-pipeline,gjtorikian/html-pipeline