Pins Symfony packages to <2.7.

[tierra]

With the latest release of 2.7 packages, Symfony bumped the minimum PHP version requirements to 5.3.9 (from 5.3.3). However, we need to still support 5.3.3, so we have to set a max version on 2.6 now.

Also performs update on each of these packages:

• symfony/console (2.6.3 to 2.6.9)
• symfony/finder (2.6.3 to 2.6.9)
• symfony/process (2.6.3 to 2.6.9)
• symfony/yaml (2.6.3 to 2.6.9)