Added explicit PHP version requirement and updated badges.

composer.json

@@ -25,6 +25,7 @@
 		}
 	},
 	"require"    : {
+		"php"                : ">=5.6",
 		"composer/installers": "~1.0",
 		"symfony/stopwatch"  : "^3.4.4",
 		"mustache/mustache"  : "~2.4",