enforce memcached version to be 2.1.0

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

The signature of the `Memcached::get()` method changed with 2.2.0.
Therefore, tests fail in Symfony when mocking `Memcached` in the
`MemcachedSessionHandlerTest` of the HttpFoundation component (see
also php-memcached-dev/php-memcached#126 and
https://bugs.php.net/bug.php?id=66331).