update travis.yml

.travis.yml

@@ -3,10 +3,10 @@ solution: ImageSharp.sln
 
 matrix:
   include:
-    - os: linux # Ubuntu 14.04
+    - os: linux # Ubuntu 16.04
-      dist: trusty
+      dist: xenial
       sudo: required
-      dotnet: 2.1.401
+      dotnet: 2.1.603
       mono: latest
 #    - os: osx # OSX 10.11
 #      osx_image: xcode7.3.1
@@ -40,4 +40,4 @@ addons:
     notification_email: james_south@hotmail.com
     build_command_prepend: "dotnet restore"
     build_command:   "dotnet build -c Release"
-    branch_pattern: coverity_scan
+branch_pattern: coverity_scan