Switch Travis-CI to use Xcode 11. (#227)

* Travis-CI: Switch MacOS to Xcode 11.
* Travis-CI: Switch ldc compiler to version 1.17.

Author: jondegenhardt

.travis.yml

@@ -20,7 +20,7 @@ matrix:
       d: dmd
       env: DUBTEST=1 MAKETEST=1 CODECOV=1
     - os: osx
-      osx_image: xcode10.2
+      osx_image: xcode11
       group: travis_latest
       language: d
       d: dmd
@@ -35,18 +35,18 @@ matrix:
       dist: xenial
       group: travis_latest
       language: d
-      d: ldc-1.16.0
+      d: ldc-1.17.0
       env: LINUX_SPECIAL=1 DEPLOY=1
       # if: type IN (pull_request, cron)
     - os: osx
-      osx_image: xcode10.2
+      osx_image: xcode11
       group: travis_latest
       language: d
-      d: ldc-1.16.0
+      d: ldc-1.17.0
       env: LTOPGO_V2=default DEPLOY=1
       # if: type IN (pull_request, cron)
     - os: osx
-      osx_image: xcode10.2
+      osx_image: xcode11
       group: travis_latest
       language: d
       d: ldc
@@ -120,7 +120,7 @@ matrix:
       env: LTOPGO_V2=default
       if: type IN (cron)
     - os: osx
-      osx_image: xcode10.2
+      osx_image: xcode11
       group: travis_latest
       language: d
       d: ldc-beta