Use the noninteractive front end for debconf

DEBIAN_FRONTEND=noninteractive tells apt-get install to use the
noninteractive front end for debconf. This keeps the system from
attempting to interact with the user during package configuration.
Without it, debconf issues warnings like:
```
debconf: unable to initialize frontend: Dialog
debconf: (TERM is not set, so the dialog frontend is not usable.)
debconf: falling back to frontend: Readline
debconf: unable to initialize frontend: Readline
debconf: (Can't locate Term/ReadLine.pm in @INC (@INC contains:
/etc/perl /usr/local/lib/perl/5.14.2 /usr/local/share/perl/5.14.2
/usr/lib/perl5 /usr/share/perl5 /usr/lib/perl/5.14 /usr/share/perl/5.14
/usr/local/lib/site_perl .) at
/usr/share/perl5/Debconf/FrontEnd/Readline.pm line 7.)
debconf: falling back to frontend: Teletype
```