Update nokogiki dependency >= 1.5

Following http://guides.rubygems.org/patterns/

A previous update forcing only Nokogiri 1.6 breaks several
applications.  Since Nokogiri 1.5 still works just fine I
modified the dependency to be >= 1.6 which accounts for
Nokogiri 1.6 as well.
Changes :uid_key to :uid_field

This brings OmniAuth-CAS more in line with other strategies like the
built-in Developer strategy, OmniAuth-Shibboleth, OmniAuth-SSO-Cookie,
etc.
Merge pull request #19 from sag47/feature_better_nokogiri_support

Update nokogiri dependency >= 1.5
Changes Nokogiri’s optimistic versioning to pessimistic

This whole pull request is essentially a revert of 577e238 and #11
Upgrades RSpec development dependency
Adjusts the nesting of the /auth/cas/callback specs

This allows them to share the same describe block
[Closes #13] Adds CAS 3.1 Single Sign Out support
Small gemspec clean-up
remove initilize and move :url attribute processing into cas.rb
Whitespace correction
Fixes some test issues with #26
Adds Ruby 2.0 as a testable version of Ruby in Travis
Issue #15 more options for CAS...

More options for values handed back by CAS.  This is needed because
different auth strategies for CAS sometimes hand back different
keys for these items.  They're not configurable on an application
basis.  Only globally.  This means omniauth-cas needs to be more
flexible and setting defaults while you're at it.
Updating documentation for new CAS options
Minor grammar change to README.
Fix option code for failing test
Updates specs to prepare for RSpec v3
Updates SimpleCov development dependency
Updates WebMock development dependency
Fixes a poorly worded line in the README