gpxlint_string: auto-detect GPX schema version
gpsbabel.t: don't harcode GPX schema version in gpxlint_string calls

gpsbabel 1.4.4 generates now GPX 1.1 files, not anymore GPX 1.0 like
in version 1.4.2. Now gpxlint_string is capable to auto-detect the
GPX schema version, use this feature.
added gpsbabel for travis-ci
.travis.yml: start a virtual X server
.travis.yml: enable long testing
.travis.yml: install some test-only CPAN modules