Fix Travis CI script

Keep only POCL builds, update POCL version to 1.2, update Ubuntu to Xenial, update
Boost version to 1.67.

Notes/TODOs:
* Fix need: When using Boost >1.67 tests fails during runtime.
* Fix needed: When using g++ 6 or 7 compilation fails.
* Add tests on Intel OpenCL Platform.

Author: jszuppe