Parallelized travis builds

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Yet another approach to running all tests in parallel (see #7708 and #8312).

This one uses [GNU Parallel](http://www.gnu.org/software/parallel/) which by default [will run one job per available CPU](http://www.gnu.org/software/parallel/man.html#jobs_n).

Comparison of random builds (recent build times on my travis account):

| PHP version| [master](https://travis-ci.org/jakzal/symfony/builds/11300678)             |  [parallel](https://travis-ci.org/jakzal/symfony/builds/11300689)
| ------------- | --- | ---
| 5.3.3| 6 min 11 sec | 3 min 45 sec
| 5.3| 7 min 26 sec | 4 min 10 sec
| 5.4| 6 min 31 sec | 3 min 31 sec	
| 5.5| 6 min 37 sec | 3 min 45 sec

On my laptop it takes 1.5min to run a whole suite parallelised (compared to over 4min when run as usual).