updated dotnet core sdk and cake build script (#717)

* updated dotnet core sdk and cake build script

* update travis SDK version

* update mono for travis and update .net core sdk to what appveyor supports

* same version of .net in travis as cake build

* same sdk on traivs and appveyor

.travis.yml

@@ -11,9 +11,9 @@ dist: trusty
 osx_image: xcode9.2
 
 mono:
-  - 4.4.2
+  - 5.10.0
 
-dotnet: 2.1.301
+dotnet: 2.1.500
 
 before_install:
   - git fetch --unshallow # Travis always does a shallow clone, but GitVersion needs the full history including branches and tags
@@ -28,4 +28,4 @@ cache:
     - .packages
     - tools/Addins
     - tools/gitreleasemanager
-    - tools/GitVersion.CommandLine
+    - tools/GitVersion.CommandLine