[Security] Replace exception mocks with actual exception instances.

It is done for two reasons:
* consistency - we use real exception objects in most of the code
* latest phpunit does not like the way we were creating mocks for exceptions (it could be also fixed by letting phpunit to call the original constructor)
removed APC on the CLI for Travis as it does not work well with PHPUnit and Composer anyway
Revert PHPUnit version, revert APC configuration
minor #10621 [Security] Replace exception mocks with actual exception instances. (jakzal)

This PR was merged into the 2.3 branch.

Discussion
----------

[Security] Replace exception mocks with actual exception instances.

It is done for two reasons:
* consistency - we use real exception objects in most of the code
* latest phpunit does not like the way we were creating mocks for exceptions (it could be also fixed by letting phpunit to call the original constructor)

Commits
-------

9438f88 [Security] Replace exception mocks with actual exception instances.
minor #10623 [2.3] Fix travis build (fabpot, romainneutron)

This PR was merged into the 2.3 branch.

Discussion
----------

[2.3] Fix travis build

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT

This PR replaces #10595 and fixes travis build on branch 2.3

Commits
-------

f613caa Revert PHPUnit version, revert APC configuration
9bdd3d1 removed APC on the CLI for Travis as it does not work well with PHPUnit and Composer anyway
updated CHANGELOG for 2.3.12
update CONTRIBUTORS for 2.3.12
updated VERSION for 2.3.12
bumped Symfony version to 2.3.13