Travis: Update minimal supported Orange version

PrimozGodec · PrimozGodec · commit b99ae8d6a31f · 2019-08-22T15:09:00.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -7,7 +7,7 @@ matrix:
   include:
 
     - python: '3.7'
-      env: ORANGE="3.20.0"
+      env: ORANGE="3.21.0"
 
     - python: '3.6'
       env: ORANGE="release"
