Fixed the booststap preprocess script to use a realtive path to mpl.rb

When trying to booststrap on stock ubuntu I ran into the following error:

    $ ./bootstrap
    ruby mplex -rmpl mp/object_callback.hmpl -o mp/object_callback.h
    /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mpl (LoadError)
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from mplex:173:in `block in <main>'
        from mplex:173:in `each'
        from mplex:173:in `<main>'

        ** preprocess failed **

This was caused by the lack of specifying a relative path to the mpl.rb file.