Bump required symfony/console version from 2.2 to 2.3
Bump required symfony/console version from 2.2 to 2.3

The current code uses Symfony Console's Command::getNativeDefinition method,
which is protected in 2.2, but public in 2.3 and newer. This was picked up by
the unit tests running against lowest deps:

https://travis-ci.org/stecman/symfony-console-completion/jobs/77916046

stecman/symfony-console-completion