remove trailing slash added by --prefix

Fixes a case where --prefix with a trailing slash (e.g. --prefix /) causes --excludes to not get picked up, for example:

    Checking path against wildcard {:path=>"/tmp/package-dir-staging20131001-14679-1as4b6o/usr/lib/python2.4/site-packages/freeswitch.py", :wildcard=>"mnt/services/freeswitch/bin/scripts", :installdir=>"/tmp/package-dir-staging20131001-14679-1as4b6o/", :level=>:debug, :file=>"fpm/package.rb", :line=>"372"}

Wouldn't exclude the directory that had to be exluded