Fix test suite

The test suite fails on travis (See the [latest build](https://travis-ci.org/sinatra/sinatra)).

This is what I did:

* require activesupport < 4.0.0 when ruby version is less than 1.9.3: activesupport is a dependency of rabl, and bundler installs the latest version, which does not support ruby 1.8.7.
* do not require nokogiri when ruby version is less than 1.9.2 (nokogiri dropped support for ruby 1.8)
* do not require redcarpet (does not support 1.8.7 anymore)

It all went well on travis.