[travis] Shorten Travis CI run times

by adding --jobs to utilize multiple cores and using boost ppa packages
instead of downloading the source and building boost from scratch.

This effectively cuts the time for a Travis CI run in half.

Note that sometimes on Travis CI `g++` may run out of memory. I haven't found a solution for that yet.
So the contributors need to weigh on faster Travis CI runs and potential false positives or slower Travis CI runs with no false positives so far.

If the latter is preferred I'll change this pull request so that `JOBS=1`.