Allow specs to run in Ruby 2.0

This commit drops support for Ruby 1.8.7, and adds it for Ruby 2.0.0.

Based on pull request (with documentation update added):

  https://github.com/causes/mock_redis/pull/58

Signed-off-by: Greg Hurrell <greg@causes.com>
Change-Id: I8617d2f860e438fc21e541a68d53afd285b1237d
Reviewed-on: https://gerrit.causes.com/24060
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>