Virtual file not found by URL - core/builtins/src/kotlin

This may be related to https://devnet.jetbrains.com/message/5522137

Since upgrading my Travis build to use IntelliJ 15.0, I'm getting the following error in my test suite -

    [junit] java.lang.AssertionError: Virtual file not found by URL: file:/home/travis/build/carymrobbins/intellij-haskforce/core/builtins/src/kotlin
    [junit]      at org.jetbrains.kotlin.idea.references.BuiltInsReferenceResolver$Companion.refreshBuiltIns(BuiltInsReferenceResolver.kt:207)

This is a bit confusing since I'm not using Kotlin, and I'm not sure how to debug this.  You can see the full stack trace here -

https://travis-ci.org/carymrobbins/intellij-haskforce/jobs/88928804#L295