[3.0] Bumped the minimum php version to 5.5

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

##### This pull request bumps the minimum php version of symfony 3.0 to php 5.5.

This also removes all the hacky workarounds for php 5.3 and 5.4.
This needs to be fixed on the 2.3 branch. That's why I haven't changed it. There is a typo there though. :)