Make Predis compatible with HHVM (at least 2.3.0).

Achieving compatibility actually required a few marginal changes:

  - HHVM still has some issues with re-entrant calls to __get(). The
    applied change is an hack simply because it is ugly, but it is not
    wrong and does not break the signature of the options interface.
  - 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.
  - In the test suite we removed an assertion for the message of an
    E_WARNING simply because HHVM emits a different message. Checking
    for the warning is actually enough in that context.

While the whole test suite passes on HHVM 2.3.0, please remember that
HHVM is still in development and things could break anytime especially
in some obscure corner cases.
Add HHVM to the build matrix on Travis CI.

We allow failures on builds run against HHVM since it is still being
developed and things could break anytime, but at least this gives us
a decent indicator of the level of compatibility of Predis with this
runtime.
Add a note about Predis on HHVM in the README.
Merge branch 'v0.9/hhvm-compatibility'