Update package.json

Make it possible to use the lib on node versions >= 6.0.0

brianc/node-postgres#1000

Author: AndyMathys

package.json

@@ -36,7 +36,7 @@
     "debug": "^2.0.0",
     "lodash": "~2.4.1",
     "moment": "~2.5.0",
-    "pg": "~4.4.3",
+    "pg": "~6.1.5",
     "pg-transaction": "^1.0.4",
     "sql": "~0.35.0"
   }