Relax development dependencies

Both Nokogiri and RSpec were pinned at a specified version. They're
relatively stable projects that don't break backwards compatibility
without a good reason, so in the interest of ensuring our dependencies
are up-to-date, relax their constraints.

Change-Id: Ia1e44ac9aa76c2322a61cd38430175b9ece3392a
Reviewed-on: http://gerrit.causes.com/36731
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>