[Cache] FilesystemAdapter checks whether the cache directory is writable

| Q                | A
| ---------------- | -----
| Bug report?      | yes
| Feature request? | no
| BC Break report? | no
| RFC?             | no
| Symfony version  | 3.1

Symfony cache component is not compatible with a read-only filesystem:
https://github.com/symfony/symfony/commit/e5d6db5c6911f9884cb79bb513053b8f12c98fda#diff-fe14fffdd6799f339b2daf1045d09360R33