minor #28114 [travis] merge "same Symfony version" jobs in one (nicolas-grekas)

This PR was merged into the 2.8 branch.

Discussion
----------

[travis] merge "same Symfony version" jobs in one

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  |
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Allowing to consume fewer jobs and save the 1 to 2 minutes bootstrap time of workers.

Commits
-------

9857ca07aa [travis] merge "same Symfony version" jobs in one

Tests/phpt/decorate_exception_hander.phpt

@@ -38,8 +38,7 @@ Did you forget a "use" statement for another namespace?"
   ["line":protected]=>
   int(%d)
   ["trace":"Exception":private]=>
-  array(0) {
-  }
+  array(%d) {%A}
   ["previous":"Exception":private]=>
   NULL
   ["severity":protected]=>