`+` operator on a depset is forbidden
`+` operator on a depset is forbidden

Fixes:

ERROR: test/proto/BUILD:36:1: in scala_proto_srcjar rule //test/proto:test_proto_java_conversions_srcjar:
Traceback (most recent call last):
	File "test/proto/BUILD", line 36
		scala_proto_srcjar(name = 'test_proto_java_conversions_srcjar')
	File "scala_proto/scala_proto.bzl", line 332, in _gen_proto_srcjar_impl
		transitive_proto_paths += target.proto.transitive_proto_path
`+` operator on a depset is forbidden. See https://docs.bazel.build/versions/master/skylark/depsets.html for recommendations. Use --incompatible_depset_union=false to temporarily disable this check.

See also #480

ianoc/rules_scala,bazelbuild/rules_scala,ianoc/rules_scala,smparkes/rules_scala,smparkes/rules_scala,smparkes/rules_scala,ianoc/rules_scala,bazelbuild/rules_scala,bazelbuild/rules_scala