adjust dependency on http-cache library

improve changelog for tag invalidation

Prepare 2.0.0 alpha (FriendsOfSymfony#360)

* Add PHP 7.1 to test matrix.
* Update changelog.
* Update FOSHttpCache dependency.
* Add php-http/message to require-dev to fix --prefer-lowest on Travis.
  Otherwise Composer throws a "Your requirements could not be resolved
  to an installable set of packages."

Merge pull request FriendsOfSymfony#338 from j0k3r/symfony-3.2-for-1.3

Add support for Symfony 3.*

Merge pull request FriendsOfSymfony#312 from FriendsOfSymfony/handle-…

…only-custom-client

[1.3] handle configuration where we only have a custom proxy client and no proxies configured

Do not sanity check hash of anonymous requests (FriendsOfSymfony#289)

Merge pull request FriendsOfSymfony#274 from bastnic/feature/hash-san…

…ity-check

check sanity of user context hash, if not, prevent setting any cache

Merge pull request FriendsOfSymfony#268 from FriendsOfSymfony/symfony-3

Symfony 3

Make tests pass on Symfony 2.8 deprecation notices

Merge pull request FriendsOfSymfony#239 from FriendsOfSymfony/symfony…

…-2.8

Test against Symfony 2.8