Use Symfony Process for running commands.

Allows for capturing stderr, timeout for runtime and time without
output.

Ensures that the environment doesn't get polluted.

And a few minor fixes.
Don't use timeouts on Travis.
Remove environment clearing.

Fails hard on Travis.
Mess minimally with composer.lock.
Updated comment.
Add comment about UNISH_NO_TIMEOUTS to phpunit.xml.dist.
Merge pull request #646 from xendk/testing-output-and-timeout

More focused error and debug output when running Drush's own phpunit tests (aka Unish). Also implements a timeout when running tests.