use yarn for external bootstrap

.travis.yml

@@ -20,9 +20,6 @@ before_install:
   - curl -o- -L https://yarnpkg.com/install.sh | bash
 
 script:
-  # Babel doesn't build correctly with npm v2 (which comes bundled with Node v4)
-  # so we manually upgrade to a known working npm version.
-  - npm install --global npm@4
   - 'if [ "$JOB" = "test" ]; then make test-ci; fi'
   - 'if [ "$JOB" = "lint" ]; then make lint && make flow; fi'
 

lerna.json

@@ -28,5 +28,7 @@
     "packages/*",
     "codemods/*",
     "experimental/*"
-  ]
+  ],
+  "npmClient": "yarn",
+  "npmClientArgs": ["--no-lockfile"]
 }