Clean up .travis.yml

- use env: global/matrix sections instead of inline scattered exports
- use official travis env var for SRC directory
- remove travis-ci default installed libhdf5 and packages that depend on them
  (mainly web database stuff like postgresql)
- install hdf5 libs and their dependencies (!) with -t explicitly from the ppa
- add some more debug information about apt-cache package policy
Merge pull request #84 from ax3l/topic/travisCleanup

Clean up .travis.yml