Install Boost 1.55 on Travis

This fixes some Travis errors:

- A recent Boost (1.55) is now installed on all configurations -> no more timeout because of Boost building locally
- Added some missing `QFile::close()`-calls -> exceptions in `TestXmlGmlReader.cpp` should be gone

I also reorganized the Travis-file structure a little bit: additional package repositories should be added before the only `sudo apt-get update`-call.