Fix connection tests skipped in PHP 7

[danielbachhuber]

In f638c97, I skipped a couple of tests that fail in PHP 7.

Example failed build https://travis-ci.org/pantheon-systems/wp-redis/builds/202458492

PHP 7.1

1) CacheTest::test_redis_reload_force_cache_flush
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'WP Redis: Redis server went away'
+''

PHP 7.0

1) CacheTest::test_redis_reload_force_cache_flush
Redis::connect(): connect() failed: Connection refused
/home/travis/build/pantheon-systems/wp-redis/tests/phpunit/test-cache.php:109
2) CacheTest::test_redis_bad_authentication
Redis::connect(): connect() failed: Connection refused

It seems like the source of these errors is in the underlying phpredis library.

I wasn't able to reproduce the PHP 7.0 error locally until I updated to phpredis 3.1.1 (https://github.com/Homebrew/homebrew-php/pull/3997), then I tried fixing it with 21963e4 which resulted in segfaults on Travis.

[danielbachhuber]

d4cbcea is another test that's now skipped.