test: fix test dependencies

related error message:

```
leanengine@3.5.0 test-tsd /home/travis/build/leancloud/leanengine-node-sdk
tsc --strict leanengine.d.ts

leanengine.d.ts:1:39 - error TS7016: Could not find a declaration file for module 'express'.
'/home/travis/build/leancloud/leanengine-node-sdk/node_modules/express/index.js' implicitly has an 'any' type.
```

Author: weakish

package.json

@@ -17,6 +17,7 @@
     "underscore": "^1.8.3"
   },
   "devDependencies": {
+    "@types/express": "^4.17.2",
     "blanket": "^1.2.3",
     "express": "^4.14.1",
     "mocha": "^3.2.0",