Update .travis.yml

.travis.yml

@@ -2,12 +2,10 @@ branches:
   except:
     - dev
 language: cpp
-matrix:
-  include:
-    - os: linux
-      dist: xenial
-    - os: osx
-      osx_image: xcode10
+os:
+  - linux
+  - osx
+dist: xenial
 addons:
   apt:
     sources:
@@ -17,6 +15,7 @@ addons:
       - pkg-config
       - libnss3-dev
   homebrew:
+    update: true
     packages:
       - ninja
       - ccache