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Make Predis v0.8 compatible with HHVM 2.4.0.

Since we cannot rely on the PHP version to detect the availability of
socket_import_stream(), we switched to function_exists(). As an added
bonus, using function_exists() is twice faster.
Update README about HHVM compatibility.
Update CHANGELOG.
Add support for key hash tags in redis-cluster (Redis 3.0.0b1).

Multi-keys operations are not allowed even when keys generate the same
hash but this will probably be supported in later betas of Redis which
means we will basically end up reusing the whole strategy used for
client-side sharding.
[tests] Adapt for testing with HHVM.
[HHVM] Work around a bug in class_exists() with non-string values.
Add HHVM to the build matrix of Travis-CI for v0.8.

Travis-CI recently provisioned virtual machines with HHVM 2.4.0 which
fixed the issues preventing Predis v0.8 from working correctly.
Add PHP 5.6 to the build matix of Travis-CI.

Builds are allowed to fail for now as PHP 5.6 is still in alpha stage.
Fix implementation for hash tags extraction from keys.

We now fully comply with the specifications defined by Redis.
fix #154
original array format
Merge remote-tracking branch 'thbourlove/server_info' into v0.8
Update CHANGELOG.
Failures for PHP 5.6 on Travis CI are not allowed anymore.
Merge remote-tracking branch 'thbourlove/v0.8' into v0.8
Update CHANGELOG and README.