Add back yarn, since changing install hook would not use yarn install…

… v1.9.4

[1/5] Validating package.json...
[2/5] Resolving packages...
success Already up-to-date.
$ node ./node_modules/vscode/bin/install
Detected VS Code engine version: ^1.25.0
Found minimal version that qualifies engine range: 1.25.0
Fetching vscode.d.ts from: https://raw.githubusercontent.com/Microsoft/vscode/0f080e5267e829de46638128001aeb7ca2d6d50e/src/vs/vscode.d.ts
vscode.d.ts successfully installed!

Done in 0.91s. anymore

.travis.yml

@@ -14,6 +14,7 @@ before_install:
     fi
 
 install:
+  - yarn
   - cd server && yarn
   - cd ..