…mfony/http-foundation to be installed.

Fixes #45

After changing the composer.json file, I ran composer update jms/serializer symfony/http-foundation so the composer.lock file is also up2date. Doing so executed the following actions:

- Installing symfony/polyfill-php70 (v1.12.0)
- Updating jms/serializer (0.16.0 => 1.14.0)
- Updating symfony/http-foundation (v2.8.34 => v3.4.32)

I checked compatibility of symfony/http-foundation and this should be safe to use until version 5.0.0, which will drop support for Symfony\Component\HttpFoundation\File\MimeType\ExtensionGuesser and Symfony\Component\HttpFoundation\File\MimeType\MimeTypeGuesser: https://github.com/symfony/http-foundation/blob/master/CHANGELOG.md#500

Those two classes are already being deprecated in Symfony 4.3 btw:

• https://github.com/symfony/http-foundation/blob/v4.3.0/File/MimeType/MimeTypeGuesser.php#L18
• https://github.com/symfony/http-foundation/blob/v4.3.0/File/MimeType/ExtensionGuesser.php#L16

I haven't checked compatibility of jms/serializer with its 1.x versions, but I think it should be safe.

Thanks!