fixing #124 so as not to use global apt cache.  This hopefully will also fix a similar issue observed when it could not get a dpkg lock.
adding architecture to the config explicitly, it is still needed on the command line however, if you config for i386 and call it on an amd64 machine it will get both sources unless you use the command line args
catching file exists more explicitly
Merge pull request #127 from ros-infrastructure/patch_124

fixing #124 so as not to use global apt cache.  This hopefully will also fix a similar issue observed when it could not get a dpkg lock.