cache web assets between CI runs (#2089)

- skip `test/jetstream.js` for `node@0.12`

test/mocha/release.js

@@ -1,4 +1,5 @@
 var assert = require("assert");
+var semver = require("semver");
 var spawn = require("child_process").spawn;
 
 if (!process.env.UGLIFYJS_TEST_ALL) return;
@@ -32,6 +33,7 @@ describe("test/benchmark.js", function() {
     });
 });
 
+if (semver.satisfies(process.version, "0.12")) return;
 describe("test/jetstream.js", function() {
     this.timeout(20 * 60 * 1000);
     [