[zanata/zanata-platform] 4ddc60: Upgrade yarn to fix palantir/tslint#3251

[GitHub]

  Branch: refs/heads/typescript
  Home:   https://github.com/zanata/zanata-platform
  Commit: 4ddc603ddbb52230280028503af5eaacd6334ba6
      https://github.com/zanata/zanata-platform/commit/4ddc603ddbb52230280028503af5eaacd6334ba6
  Author: Sean Flanigan <sflaniga at redhat.com>
  Date:   2018-01-31 (Wed, 31 Jan 2018)

  Changed paths:
    M server/zanata-frontend/pom.xml
    R server/zanata-frontend/src/.yarn-version
    M server/zanata-frontend/src/package.json
    A server/zanata-frontend/src/scripts/check-engines.js
    M server/zanata-frontend/src/yarn.lock

  Log Message:
  -----------
  Upgrade yarn to fix palantir/tslint#3251