message: [c# travis] Replace deprecated xbuild with msbuild
  Switch Travis to use MSBuild instead of XBuild because it's deprecated.
diff: @@ -72,8 +72,8 @@ script:
  - if [ "$FLAVOR" == "cs" ]; then make DESTDIR=$HOME install; fi
  - if [ "$FLAVOR" == "cs" ]; then cd ..; fi
  - if [ "$FLAVOR" == "cs" ]; then export BOND_COMPILER_PATH=$HOME/usr/local/bin; fi
  - - if [ "$FLAVOR" == "cs" ]; then xbuild /p:Configuration=Debug cs/cs.sln; fi
  - - if [ "$FLAVOR" == "cs" ]; then xbuild /p:Configuration=Fields cs/cs.sln; fi
  + - if [ "$FLAVOR" == "cs" ]; then msbuild /p:Configuration=Debug cs/cs.sln; fi
  + - if [ "$FLAVOR" == "cs" ]; then msbuild /p:Configuration=Fields cs/cs.sln; fi
  - if [ "$FLAVOR" == "cs" ]; then mono NUnit.Runners.2.6.4/tools/nunit-console.exe -framework=mono-4.5 -labels cs/test/core/bin/debug/net45/Properties/Bond.UnitTest.dll cs/test/core/bin/debug/net45/Fields/Bond.UnitTest.dll cs/test/internal/bin/debug/net45/Bond.InternalTest.dll; fi
  - if [ "$FLAVOR" == "cpp-core" ]; then cmake -DBOND_SKIP_GBC_TESTS=TRUE -DBOND_ENABLE_GRPC=FALSE ..; fi
  - if [ "$FLAVOR" == "cpp-core" ]; then make --jobs 2 check; fi
  