Loosen gem dependencies.

Hello, I loosened the gems dependencies a bit.

Newer prawn versions ( >= 0.13, <= 0.15) have a ~> 1.2 dependency on pdf-reader so I was unable to use pdf-reader-turtletext on my project.
I ran the specs with pdf-reader 1.2 against ruby 1.9.3 and 2.0.0 without any problem.

So I changed the pdf-reader dependecy to: >= 1.1.1, <= 1.2

Thanks. Great gem, btw.