title: Strange build failure when deploying GAE using gradle
body: <p>Everything used to work fine until today. Didn't change anything as far as I know and now I get this:</p>
  
  <pre><code>C:\mypath&gt;gradle appengineDeploy
  
  &gt; Configure project :
  WARNING: You are a using release candidate 2.0.0-rc1. Behavior of this plugin has changed since 1.3.5. Please see release notes at: https://github.com/GoogleCloudPlatform/app-gradle-plugin.
  Missing a feature? Can't get it to work?, please file a bug at: https://github.com/GoogleCloudPlatform/app-gradle-plugin/issues.
  
  &gt; Task :appengineDeploy FAILED
  
  FAILURE: Build failed with an exception.
  
  * What went wrong:
  Execution failed for task ':appengineDeploy'.
  &gt; Deployment version must be defined or configured to read from system state
    1. Set appengine.deploy.version = 'my-version'
    2. Set appengine.deploy.version = 'GCLOUD_CONFIG' to have gcloud generate a version for you.
    3. Using APPENGINE_CONFIG is not allowed for flexible environment projects
  
  * Try:
  Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
  
  * Get more help at https://help.gradle.org
  
  Deprecated Gradle features were used in this build, making it incompatible with Gradle 5.0.
  See https://docs.gradle.org/4.8.1/userguide/command_line_interface.html#sec:command_line_warnings
  
  BUILD FAILED in 6s
  8 actionable tasks: 6 executed, 2 up-to-date
  </code></pre>
  
  <p>I just updated gradle from version 4.5.1 to 4.8.1 but the same issue remains. I understand it complains about version of the appengine but I never had to state that before so I think it's due to some update on Google's side. What am I missing? </p>