Fix travis build

Author: Cory Thompson

.travis.yml

@@ -7,7 +7,4 @@ install:
   - dotnet restore
 
 script:
-  - dotnet build --framework=netcoreapp1.0
-  - dotnet build --framework=netcoreapp1.1
-  - dotnet build --framework=netcoreapp2.0
   - dotnet test --framework=netcoreapp2.0 WebPush.Test/WebPush.Test.csproj