(MAINT) update nokogiri dependency

- Fixes: Unable to resolve dependencies: aws-sdk requires nokogiri (>= 1.4.4);
  rbvmomi requires nokogiri (>= 1.4.1); beaker requires nokogiri (=
  1.5.10); fog requires nokogiri (>= 1.5.11, ~> 1.5)
Merge pull request #307 from anodelman/maint

(MAINT) update nokogiri dependency