Updating to dotnet 5.0 (#1218)

* adding network file validator contract

* updating to dotnet 5

* reverting change in pipeline

* updating target framework to 5.0

* updating srgen and using target framework var

* reverting srgen back to 3.1

* Adding dotnet 3.1 to build

* Fixing task def

* removing global json

* updating other 3.1 refs to 5.0

* removing netcore folder from build.cake

* Moving and renaming dotnet 3.1 task

.travis.yml

@@ -7,7 +7,7 @@ os:
 #  - osx
 
 mono: none
-dotnet: 3.1
+dotnet: 5.0
 
 # safelist
 branches:
@@ -25,7 +25,7 @@ env:
 before_install:
   - if [ $TRAVIS_OS_NAME == "linux" ]; then
       sudo apt-get update;
-      sudo apt-get install dotnet-sdk-3.1;
+      sudo apt-get install dotnet-sdk-5.0;
     else
       brew update;
     fi