I reverted back to 12.0.1
implementation 'com.google.firebase:firebase-ads:12.0.1'
Seems to me that Google need to fix this their end.
Update 4 May 2018:
Firebase-15.0.1 depends on the latest google services
classpath 'com.google.gms:google-services:3.3.0'
See this blog post
However when I upgraded google services 3.3.0 and firebase-15.0.1 I ran into other issues that I couldn't resolve. Using Google-Services 3.3.0, gradle complained about my library module that I imported:
implementation project(':lib')
Gradle Error:
More than one variant of project :lib matches the consumer attributes:
- Configuration ':lib:debugApiElements' variant android-aidl:
- Found artifactType 'android-aidl' but wasn't required.
- Required com.android.build.api.attributes.BuildTypeAttr 'debug' and found compatible value 'debug'.
- Found com.android.build.api.attributes.VariantAttr 'debug' but wasn't required.
- Required
com.android.build.gradle.internal.dependency.AndroidTypeAttr 'Aar' and found compatible value 'Aar'.
- Required org.gradle.usage 'java-api' and found compatible value
So again my recommendation is not to upgrade.