Update .travis.yml

Author: vincentgeneste

.travis.yml

@@ -7,7 +7,7 @@ dist: trusty
 sudo: required
 
 mono: none
-dotnet: 2.0.0
+dotnet: 3.1.0
 
 before_install:
   - cd Phantasma.Tests
@@ -23,4 +23,4 @@ script:
 after_failure:
   - wget https://raw.githubusercontent.com/DiscordHooks/travis-ci-discord-webhook/master/send.sh
   - chmod +x send.sh
-  - ./send.sh failure $WEBHOOK_URL
+  - ./send.sh failure $WEBHOOK_URL