Fix artifact upload for compiler

check-artifact.sh was broken by the update to Gradle 2.10. We think the
update to Gradle 2.8 caused the POM to start being generated, but this
now fixes it to have the correct contents.

Using addFilter _disables_ the normal POM, so we use setFilter instead.

Fixes #1360
diff --git a/compiler/check-artifact.sh b/compiler/check-artifact.sh
index 7dbd259..a915e2a 100755
--- a/compiler/check-artifact.sh
+++ b/compiler/check-artifact.sh
@@ -125,5 +125,5 @@
   echo
 }
 
-FILE="build/artifacts/java_pluginExecutable/protoc-gen-grpc-java.exe"
+FILE="build/artifacts/java_plugin/protoc-gen-grpc-java.exe"
 checkArch "$FILE" && checkDependencies "$FILE"