Set default debian install_opts to accommodate upgrades.

Without setting this, the cookbook as-is wont accommodate upgrades.
Default behavior of dpkg is to prompt when new package files will
clobber existing package files which have seen changes.  While apt
can have a global setting tuned to reflect similar install options,
this just assumes that you want to get to the declared version no
matter what state you might be in previously.
Merge pull request #8 from filler/add_debian_install_opts

Set default debian install_opts to accommodate upgrades.