Kept receving a:
```
Fetching source index from http://rubygems.org/
Resolving dependencies...
Bundler could not find compatible versions for gem "nokogiri":
  In Gemfile:
    quickeebooks (>= 0) ruby depends on
      nokogiri (~> 1.5.9) ruby

    nokogiri (1.6.1)
```
.. when trying to bundle in a client project
after updating quickbooks-ruby to the latest. Removing the
pessimistic constraint allowed the bundle to go thru fine.

Let's roll back this change as many apps are going to be running
quickeebooks also.
Merge pull request #41 from minimul/nokogiri

Kept receving a: