Allow failures in Travis configuration

Author: martinmoene

.travis.yml

@@ -152,6 +152,13 @@ matrix:
       compiler: clang
       env: COMPILER='clang++'
 
+  allow_failures:
+    - env: COMPILER=clang++-5.0
+    - env: COMPILER=clang++-6.0
+    - osx_image: xcode10
+
+  fast_finish: true
+  
 script:
   - export CXX=${COMPILER}
   - JOBS=2 # Travis machines have 2 cores.