Clean up .travis.yml

- use **env**: *global/matrix* sections instead of inline scattered `export`'s
- 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*