only run flow check in CI if npm 3

scripts/run-flow-check.js

@@ -0,0 +1,15 @@
+'use strict';
+
+var binVersionCheck = require("bin-version-check");
+var child = require("child_process");
+
+// run in npm 3+ only
+binVersionCheck("npm", ">=3.3.0", function (err) {
+  if (!err) {
+    console.log("Running ./node_modules/.bin/flow check");
+    var cmd = child.spawn("./node_modules/.bin/flow", ["check"], { stdio: "inherit" });
+    cmd.on("exit", function(code) {
+      console.log(code);
+    });
+  }
+});