[loop-libev] Add libev as require-dev and suggest (in event-loop)

Author: igorw

composer.json

@@ -7,7 +7,8 @@
         "php": ">=5.3.3"
     },
     "suggest": {
-        "ext-libevent": ">=0.0.5"
+        "ext-libevent": ">=0.0.5",
+        "ext-libev": "*"
     },
     "autoload": {
         "psr-0": { "React\\EventLoop": "" }