Revert "no longer need duplicate expect declaration"

This reverts commit e9aa9a3.

This brings back TypeScript definition that is necessary for
Cypress 1.1.4 to pass TS linter

Author: bahmutov

examples/preprocessors__typescript-webpack/cypress/integration/spec.ts

@@ -1,5 +1,8 @@
 import { add } from '../support/add'
 
+// brings type definition from @types/chai
+declare const expect: Chai.ExpectStatic
+
 describe('TypeScript', () => {
   it('works', () => {
     // note TypeScript definition