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.