minor symfony#17196 [travis] Use xargs instead of parallel to test --enable-sigchild (nicolas-grekas)

This PR was merged into the 3.1-dev branch.

Discussion
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[travis] Use xargs instead of parallel to test --enable-sigchild

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

The --timeout option doesn't work, jobs still fail with a 10 minutes timeout...
Let's see the output with xargs, if it happens again...

Commits
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f3def51 [travis] Use xargs instead of parallel to test --enable-sigchild

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