Update deployment instructions to reference yarn

Author: ChadKillingsworth

deployments.md

@@ -1,12 +1,14 @@
 # Deploying Closure Compiler to NPM
 
+*You now need yarn installed - `npm install -g yarn`*
+
  1. Update the compiler submodule pointer to the tagged release.
      * change to the '/compiler' folder
      * `git checkout` the correct tag/commit
      * change back to the root folder and commit this change
  2. Update the package version number in `package.json`. The major version should be equal to the compiler version
     and the minor and patch numbers should be `0`. Example: 20160619.0.0.
- 3. Run the tests: `npm install && npm test`.
+ 3. Run the tests: `yarn install && yarn test`.
  3. Commit the changes from steps 1 & 2 and tag the commit.
  4. Push the changes and tag. If the tests all pass, Travis-CI will automatically publish the new release to the
    npm registry.