[3.0] Rename Symfony2 to Symfony

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |  n/a
| License       | MIT

I did not rename Symfony BrowserKit HTTP user agent as it might break BC.