added DB_File as a prereq

Usually installed with Perl itself, but may be missing, e.g.
with self-compiled Perls (perlbrew! travis!).
first steps for travis testing

* don't prereq "difficult" modules like Image::Magick or X11::Protocol
* use "test-no-cgi" target
added .travis.yml
added Geo::Proj4 to list of uninstallable dependencies
travis build: removed 5.8 because of DBD::XBase issues
travis: added GD and dependencies to the list of uninstallable dependencies
travis: install deps via apt-get, call "make ext"
more system prereqs needed for Geo::Proj4 and DB_File under Ubuntu
travis.yml: added agrep to list of pre-installed packages
.travis.yml: added reasons for installing packages
.travis.yml: installing GD should work now