Updates gemspec to use pessimistic versioning on dependencies

Let's talk about versioning.

The majority of ruby gems use (and the rest should use) the [Rational Versioning Policy](http://docs.rubygems.org/read/chapter/7) defined by RubyGems.org.

Currently, `active_merchant` has several sub-optimal version definitions on its dependencies. Things like

    s.add_dependency('active_utils', '>= 1.0.2')

and

    s.add_dependency('nokogiri', "< 1.6.0")

Since patch and minor version changes should not break backwards compatibility, let's ditch these and instead start using [pessimistic versioning](http://docs.rubygems.org/read/chapter/16#page74) in all applicable cases. The above examples become

    s.add_dependency('active_utils', '~> 1.0')
    s.add_dependency('nokogiri', "~> 1.6")

Much cleaner.

I've updated the gemspec to use pessimistic versioning for all of the runtime dependencies. I've left the development dependencies alone for now as I'm not sure how best to proceed there.