Added Symfony4 constraint

Merge pull request #26 from Evgen14/0.3.x

Add symfony `4.0`

0.3.3

• Fixed minor bugs due to Symfony next versions

0.3.2

• Target path fixes
• PHPUnit test fixes

0.3.1

• Minor docs fixes
• Added public method for getting trusted target path from session.

0.3.0

• Symfony ~2.8|~3.0

0.2.3

• Added new class SsoEvents with SSO_AUTHORIZED event.
• Extracted some code from controller to the new service krtv_single_sign_on_identity_provider.security.http_utils

0.2.2

• Fixed bug with logout when logout started from IdP

Release 0.2.1

• _target_path is now configurable #6

Release 0.1.1

• Ported tests from 0.2.x branch
• _target_path is now configurable #6

Release 0.2.0

• Some refactoring
• Added a lot of tests
• Added configuration examples and test IdP app.