Fix RubyMotion compatibility issue

When I used this library with RubyMotion, I got the following error when compiling the code:

    Objective-C stub for message `identiconWithString:size:backgroundColor:' type `@@:@f@' not precompiled.
    Make sure you properly link with the framework or library that defines this message.

Importing `CoreGraphics/CGBase.h` fixed the problem. I also updated the `README` to specify that the `CoreGraphics` framework is required.