Fix extension build so that gem installation uses the same path as the rake test.
Currently when you attempt to load the installed gem, you get the following error:

    /lib/prawn/gmagick.rb:2:in `require_relative': cannot load such file -- ...bundler/gems/prawn-gmagick-181cfeb75911/lib/prawn/gmagick/image (LoadError)

because the extension compiles into lib/image.bundle rather than lib/prawn/gmagick/image.bundle like the Rake task triggered under testing used. This patch standardizes on the default gem installation behavior.
Don't have a runtime dependency on rake-compiler.
Merge pull request #7 from jcoleman/fix-extension-build

Build extension in correct location