title: What can break an android build without code changes?
body: <p>I am working on a <code>react-native</code> app that uses <a href="https://rnfirebase.io/" rel="nofollow noreferrer">react-native-firebase</a>. I've setup a CI build on travis with the aim to have reproducible builds, and it has suddenly stopped working, as detailed here:</p>
  
  <ul>
  <li><a href="https://travis-ci.org/mapswipe/mapswipe/jobs/545759097" rel="nofollow noreferrer">before</a>: 4 days ago, the build works</li>
  <li><a href="https://travis-ci.org/mapswipe/mapswipe/jobs/545759086#L3002" rel="nofollow noreferrer">after</a>: today, the exact same build is broken, failing with the following message:</li>
  </ul>
  
  <blockquote>
    <p>Android dependency 'com.google.android.gms:play-services-base' has different version for the compile (16.0.1) and runtime (17.0.0) classpath. You should manually set the same version via DependencyResolution</p>
  </blockquote>
  
  <p>My only reasonable guess is that some dependency has changed, but I don't understand android builds well enough to figure out what has happened or why.</p>
  
  <p>And obviously, how do I prevent such surprises from happening in the future? (in my mind, this build should be perfectly repeatable)</p>
  
  <p>As a side note, when the build fails, it also shows several warnings like:</p>
  
  <blockquote>
    <p>WARNING: Conflict with dependency 'com.google.android.gms:play-services-base' in project ':app'. Resolved versions for runtime classpath (17.0.0) and compile classpath (16.0.1) differ. This can lead to runtime crashes. To resolve this issue follow advice at <a href="https://developer.android.com/studio/build/gradle-tips#configure-project-wide-properties" rel="nofollow noreferrer">https://developer.android.com/studio/build/gradle-tips#configure-project-wide-properties</a>. Alternatively, you can try to fix the problem by adding this snippet to /home/travis/build/mapswipe/mapswipe/android/app/build.gradle:
    dependencies {
        implementation("com.google.android.gms:play-services-base:17.0.0")
    }</p>
  </blockquote>
  
  <p>but following that advice only causes further (different) errors.</p>