building pre-release versions

(trying out https://github.com/travis-ci/dpl/issues/234#issuecomment-123309684)

.travis.yml

@@ -11,6 +11,7 @@ notifications:
     on_start: false
 deploy:
   provider: releases
+  prerelease: true
   api_key:
     secure: ZjE1j93v3qbPIe2YbmhS319aCbMdLQw0HuymmluTurxXsZtn9D4t2+eTr99vBVxGRuB5lzzGezPR5zjk5W7iHF7xhwrawXrFzr2rPJWzWFt0aM+Ry2njU1ROTGGXGTbv4anWeBlgMxLEInTAy/9ytOGNJlec83yc0THpOY2wxnk=
   file: main/target/Cryptomator-$TRAVIS_TAG.jar