Work around travis bug by forcing a homebrew update

.travis.yml

@@ -14,7 +14,7 @@ notifications:
 matrix:
   include:
     - os: osx
-      osx_image: xcode10.2 # for c++17
+      osx_image: xcode11.3 #xcode10.2 # for c++17
       compiler: clang
       addons:
         homebrew:
@@ -22,6 +22,7 @@ matrix:
             - clang-format
             - ninja
             - openssl
+          update: true
     - os: linux
       compiler: gcc
       env: