travis: Make macOS' CI use Xcode 10

Keeps the toolchain used for CI up to date. Xcode 10 also brings in
better (but not complete) C++17 support on macOS. Notably it allows the
use of the <any>, <optional>, and <variant> headers that were introduced
in C++17, among other things. Now we can use the standard-provided
facilities instead of needing to rely on boost for these.

.travis.yml

@@ -36,7 +36,7 @@ matrix:
     - os: osx
       env: NAME="macos build"
       sudo: false
-      osx_image: xcode9.3
+      osx_image: xcode10
       install: "./.travis/macos/deps.sh"
       script: "./.travis/macos/build.sh"
       after_success: "./.travis/macos/upload.sh"