Fix bug when intl extension is not installed

If a server has not the php intl extension, symfony use symfony/intl as
a replacement layer, it works only for english language and do not
implement all intl's features.
In this case, DateType call `\Locale::getPrimaryLanguage()` and throw an
Exception. This commit prevent this Exception.
Merge pull request #346 from WedgeSama/intl-fix

Fix bug when intl extension is not installed