Support symfony/deprecation-contracts v3

[pkruithof]

v3.0 was released 8 days ago. When updating composer, it downgraded Panther because it doesn't support it yet:

/app # composer update
Loading composer repositories with package information
Updating dependencies
Lock file operations: 0 installs, 4 updates, 0 removals
  - Upgrading symfony/deprecation-contracts (v2.5.0 => v3.0.0)
  - Downgrading symfony/panther (v1.1.1 => v1.0.1)

[chalasr]

Apologies, we missed your PR. Done in #519. Thank you!

[pkruithof]

No worries, as long as it's done!