Change to "--framework netcoreapp2.1"

[BruceHaley]

Turns out netcoreapp3.0 creates a test bot that does not work. Specifying 2.1 does work.

[coveralls]

Pull Request Test Coverage Report for Build 98959

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.6%) to 77.829%

---

Totals
Change from base Build 98951:	-1.6%
Covered Lines:	20774
Relevant Lines:	26692

---

💛 - Coveralls

[fuselabs]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.Luis.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.QnA.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.ApplicationInsights.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Azure.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Dialogs.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.ApplicationInsights.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.AspNet.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.TemplateManager.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Testing.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Configuration.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Connector.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Schema.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Streaming.dll compared against version 4.6.3