minor symfony#17151 [travis] timeout the sigchild tests at 60s (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
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[travis] timeout the sigchild tests at 60s

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

[Sometimes](https://travis-ci.org/symfony/symfony/jobs/98790028#L2774), the tests of the process component with sigchild enabled are terminated by travis after 10 minutes of no activity.
In order to free the failing job faster and to get some output, let's kill the job ourselves after 60s (running the test suite usually takes ~8s on travis)

Commits
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f78e794 [travis] timeout the sigchild tests at 60s

Author: nicolas-grekas

.travis.yml

@@ -59,7 +59,7 @@ install:
 script:
     - if [[ ! $deps ]]; then echo "$COMPONENTS" | parallel --gnu '$PHPUNIT --exclude-group tty,benchmark,intl-data {}'; fi;
     - if [[ ! $deps ]]; then echo -e "\\nRunning tests requiring tty"; $PHPUNIT --group tty; fi;
-    - if [[ ! $deps && $TRAVIS_PHP_VERSION = ${MIN_PHP%.*} ]]; then echo -e "1\\n0" | parallel --gnu 'echo -e "\\nPHP --enable-sigchild enhanced={}" && ENHANCE_SIGCHLD={} php-$MIN_PHP/sapi/cli/php .phpunit/phpunit-4.8/phpunit --colors=always src/Symfony/Component/Process/'; fi;
+    - if [[ ! $deps && $TRAVIS_PHP_VERSION = ${MIN_PHP%.*} ]]; then echo -e "1\\n0" | parallel --gnu --timeout 60 'echo -e "\\nPHP --enable-sigchild enhanced={}" && ENHANCE_SIGCHLD={} php-$MIN_PHP/sapi/cli/php .phpunit/phpunit-4.8/phpunit --colors=always src/Symfony/Component/Process/'; fi;
     - if [[ $deps = high ]]; then echo "$COMPONENTS" | parallel --gnu -j10% 'cd {}; composer --prefer-source update; $PHPUNIT --exclude-group tty,benchmark,intl-data'$LEGACY; fi;
     - if [[ $deps = low ]]; then echo "$COMPONENTS" | parallel --gnu -j10% 'cd {}; composer --prefer-source --prefer-lowest --prefer-stable update; $PHPUNIT --exclude-group tty,benchmark,intl-data'; fi;
     - if [[ $deps = skip ]]; then echo This matrix line is skipped for pull requests.; fi;