Merge pull request #474 from jeremymeng/pin-types-express-serve-static-core

as later patch versions introduce string interpolations that are not recognized
by typescript v3.

Upgrading typescript is another option, however, that introduces incompatibility
with tslint.

lib/util/constants.ts

@@ -7,7 +7,7 @@ export const Constants = {
    * @const
    * @type {string}
    */
-  msRestVersion: "2.6.2",
+  msRestVersion: "2.6.3",
 
   /**
    * Specifies HTTP.

package.json

@@ -5,7 +5,7 @@
     "email": "azsdkteam@microsoft.com",
     "url": "https://github.com/Azure/ms-rest-js"
   },
-  "version": "2.6.2",
+  "version": "2.6.3",
   "description": "Isomorphic client Runtime for Typescript/node.js/browser javascript client libraries generated using AutoRest",
   "tags": [
     "isomorphic",
@@ -66,7 +66,8 @@
     "@microsoft/api-extractor": "^7.18.11",
     "@ts-common/azure-js-dev-tools": "^19.4.0",
     "@types/chai": "^4.1.7",
-    "@types/express": "^4.17.0",
+    "@types/express": "4.17.0",
+    "@types/express-serve-static-core": "4.17.0",
     "@types/fetch-mock": "^7.3.1",
     "@types/form-data": "^2.2.1",
     "@types/glob": "^7.1.1",