Change gcc-multilib to recommends in deb-packages

The gcc-multilib package is not required for dmd to work on 64-bit
systems when 32-bit software is not going to be built. Changing from
'Depends' to 'Recommends' still suggest a strong connection but it will
be possible to choose not to install gcc-multilib.
Merge pull request #77 from andron/gccmultilib

Change gcc-multilib to recommends in deb-packages