Upgrade to .NET Core SDK 2.1.200

Merge of PR #512

Author: atifaziz

.travis.yml

@@ -7,7 +7,7 @@ solution: MoreLinq.sln
 mono: 5.0.1
 dist: trusty
 sudo: required
-dotnet: 2.1.2
+dotnet: 2.1.200
 env:
   - CONFIGURATION=Debug
   - CONFIGURATION=Release

global.json

@@ -4,6 +4,6 @@
         "MoreLinq.Test"
     ],
     "sdk": {
-        "version": "2.1.2"
+        "version": "2.1.200"
     }
 }