add Diffractor to integration tests (#532)

Author: simeonschaub

.github/workflows/IntegrationTest.yml

@@ -17,6 +17,11 @@ jobs:
         package:
           - {user: dpsanders, repo: ReversePropagation.jl}
           - {user: FluxML, repo: Zygote.jl}
+        # Diffractor needs to run on Julia nightly
+        include:
+          - julia-version: nightly
+            os: ubuntu-latest
+            package: {user: JuliaDiff, repo: Diffractor.jl}
 
     steps:
       - uses: actions/checkout@v2