update travis file

Author: Courio-Dev

.travis.yml

@@ -3,7 +3,7 @@ os:
   - osx
   - linux
 
-# Ubuntu 14.04
+# .NET CLI require Ubuntu 14.04
 sudo: required
 dist: trusty
 addons:
@@ -16,19 +16,19 @@ addons:
     - libunwind8
     - zlib1g
 
-# OS X 10.12
-osx_image: xcode9.2
-
-# Ensure Mono is installed
-mono:
-  - latest
+# .NET CLI requires OSX 10.11
+osx_image: xcode7.2
 
+# Ensure that .NET Core is installed
+dotnet: 2.1.105
 # dotnet: 2.1.300-preview1-008174
-dotnet: 2.1.4
 # sudo apt-get install apt-transport-https
 # sudo apt-get update
 # sudo apt-get install dotnet-sdk-2.1.300-preview2-008533
 
+# Ensure Mono is installed
+mono: latest
+
 env:
     # Set the DOTNET_SKIP_FIRST_TIME_EXPERIENCE environment variable to stop wasting time caching packages
   - DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true