message: [travis c++] Compile with Boost 1.65.1 and GCC
  Switch to building some configurations with GCC. This is part of a larger
  set of changes that will build some representative configurations for each
  PR and a fuller matrix out-of-band.
  Issue has more details.
diff: @@ -13,18 +13,18 @@ services:
  - docker
  env:
  - FLAVOR="cs"
  - - FLAVOR="cpp-core" BOOST="1.65.1"
  - - FLAVOR="cpp-core" BOOST="1.64.0"
  - - FLAVOR="cpp-core" BOOST="1.63.0"
  - - FLAVOR="cpp-core" BOOST="1.62.0"
  - - FLAVOR="cpp-core" BOOST="1.61.0"
  - - FLAVOR="cpp-core" BOOST="1.60.0"
  - - FLAVOR="cpp-core" BOOST="1.59.0"
  - - FLAVOR="cpp-core" BOOST="1.58.0"
  - - FLAVOR="cpp-comm" BOOST="1.63.0"
  - - FLAVOR="cpp-grpc" BOOST="1.65.1"
  - - FLAVOR="cpp-grpc" BOOST="1.64.0"
  - - FLAVOR="cpp-grpc" BOOST="1.63.0"
  + - FLAVOR="cpp-core" BOOST="1.65.1" COMPILER="gcc"
  + - FLAVOR="cpp-core" BOOST="1.64.0" COMPILER="clang"
  + - FLAVOR="cpp-core" BOOST="1.63.0" COMPILER="clang"
  + - FLAVOR="cpp-core" BOOST="1.62.0" COMPILER="clang"
  + - FLAVOR="cpp-core" BOOST="1.61.0" COMPILER="clang"
  + - FLAVOR="cpp-core" BOOST="1.60.0" COMPILER="clang"
  + - FLAVOR="cpp-core" BOOST="1.59.0" COMPILER="clang"
  + - FLAVOR="cpp-core" BOOST="1.58.0" COMPILER="clang"
  + - FLAVOR="cpp-comm" BOOST="1.63.0" COMPILER="clang"
  + - FLAVOR="cpp-grpc" BOOST="1.65.1" COMPILER="gcc"
  + - FLAVOR="cpp-grpc" BOOST="1.64.0" COMPILER="clang"
  + - FLAVOR="cpp-grpc" BOOST="1.63.0" COMPILER="clang"
  - FLAVOR="hs"
  before_install:
  @@ -35,4 +35,4 @@ before_install:
  - time docker load -i $HOME/docker-images/bond-xenial.tar.gz
  script:
  - - docker run -v $HOME/.ccache:/root/.ccache -v $HOME/.stack:/root/.stack -v `pwd`:/root/bond bond-xenial $HOME $FLAVOR $BOOST
  + - docker run -v $HOME/.ccache:/root/.ccache -v $HOME/.stack:/root/.stack -v `pwd`:/root/bond bond-xenial $HOME $FLAVOR $BOOST $COMPILER
  