message: [travis] Test grpc on clang
  We've found that Clang finds more issues than GCC does in the gRPC integration code.
diff: @@ -14,7 +14,7 @@ services:
  env:
  - FLAVOR="cs"
  - FLAVOR="cpp-core" BOOST="1.66.0" COMPILER="clang"
  - - FLAVOR="cpp-grpc" BOOST="1.66.0" COMPILER="gcc"
  + - FLAVOR="cpp-grpc" BOOST="1.66.0" COMPILER="clang"
  - FLAVOR="hs"
  - FLAVOR="java"
  