Mention ttl units in docs

As far as I can tell, `ttl` is directly subtracted from time() in Gaufrette, which means it's in seconds. Good idea to document.

Resources/docs/adapters/cache.md

@@ -6,7 +6,7 @@ Adapter which allows you to cache other adapters
 
  * `source` The source adapter that must be cached *(required)*
  * `cache` The adapter used to cache the source *(required)*
- * `ttl` Time to live *(default 0)*
+ * `ttl` Time to live, in seconds *(default 0)*
  * `serializer` The adapter used to cache serializations *(default null)*
 
 ## Example
@@ -35,4 +35,4 @@ knp_gaufrette:
     filesystems:
         media:
             adapter: media_cache
-```
+```

composer.json

@@ -15,15 +15,9 @@
             "homepage": "https://github.com/knplabs/KnpGaufretteBundle/contributors"
         }
     ],
-    "repositories": [
-        {
-            "type": "git",
-            "url": "https://github.com/zyphlar/Gaufrette"
-        }
-    ],
     "require":      {
         "symfony/framework-bundle": "~2.0|~3.0",
-        "knplabs/gaufrette":        "dev-sw"
+        "knplabs/gaufrette":        "~0.1.7|~0.2"
     },
     "require-dev": {
         "symfony/yaml": "~2.0|~3.0",