Speed up travis builds by skipping the .net core initialisation (#1755)

Author: bergmeister

.travis.yml

@@ -3,6 +3,10 @@ language: cpp
 git:
   depth: 1000
 
+env:
+  # Avoid expensive initialization of dotnet cli, see: https://donovanbrown.com/post/Stop-wasting-time-during-NET-Core-builds
+  DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1
+
 os:
   - linux
   - osx