ignorefile is not versioned in the Builderator gemspec

[fmitchell-r7]

When running gem build builderator.gemspec I get the following error:

WARNING:  open-ended dependency on ignorefile (>= 0) is not recommended
  if ignorefile is semantically versioned, use:
    add_runtime_dependency 'ignorefile', '~> 0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for help

We should pin ignorefile at a version in the gemspec.