Removed all extension dependencies, and instead added them as suggestions.

Author: magnusnordlander

composer.json

@@ -25,7 +25,6 @@
     ],
     "require":           {
         "php":                      "^5.5|^7.0",
-        "ext-memcached":            "*",
         "psr/cache":                "^1.0",
         "cache/adapter-common":     "^0.3",
         "cache/taggable-cache":     "^0.4",
@@ -35,6 +34,9 @@
         "phpunit/phpunit":         "^4.0|^5.1",
         "cache/integration-tests": "^0.9"
     },
+    "suggest":          {
+        "ext-memcached": "The extension required to use this pool."
+    },
     "provide":           {
         "psr/cache-implementation": "^1.0"
     },