Fix extension build

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.

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Also eliminates the runtime dependency on rake-compiler.