[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 the password_hash workarounds for older versions of php.
@TomasVotruba `~2.2.0` is a different version constraint to `~2.2`. They behave totally differently. Keeping the `.0` in the all php version constraints seems consistent to me.