make travis mark releases as prerelease

Martin Müllenhaupt · Martin Müllenhaupt · commit b5f6404215ce · 2017-06-09T11:25:36.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -100,3 +100,4 @@ deploy:
   skip_cleanup: true
   on:
     tags: true
+  prerelease: true
