Upgrade the Travis config.

Found at Jon Skeet's blog:
https://codeblog.jonskeet.uk/2017/08/15/using-net-core-2-0-sdk-on-travis/

.travis.yml

@@ -5,11 +5,19 @@ env:
 
 sudo: required #false
 dist: trusty
-dotnet: 1.0.0-preview2-003121
-
+dotnet: 2.0.0
 mono:
 - latest
 
+addons:
+  apt:
+    sources:
+    - sourceline: 'deb [arch=amd64] https://packages.microsoft.com/repos/microsoft-ubuntu-trusty-prod trusty main'
+      key_url: 'https://packages.microsoft.com/keys/microsoft.asc'
+    packages:
+    - dotnet-hostfxr-1.0.1
+    - dotnet-sharedframework-microsoft.netcore.app-1.0.5
+
 branches:
   except:
     - gh-pages