Few improvements to Travis CI config:

- it's better to specify language and compiler explicitly;
- removed duplicate `libqt4-dev` dependency;
- no need in `debhelper` dependency, unless we would like to test .deb
  package generation too;
- it seems that there is no need in `pkg-config` dependency;
- no need to run `qmake -project` as we already have a .pro file.