Update .travis.yml

Author: FabianTerhorst

.travis.yml

@@ -5,7 +5,7 @@ filter_secrets: false
 sudo: false
 language: cpp
 env:
-  - GITHUB_DEPLOY_PRERELEASE=$(if echo $VERSION | grep -q "\-" ; then true; else false; fi)
+  - GITHUB_DEPLOY_PRERELEASE=$(if echo $VERSION | grep -q "\-" ; then echo true; else echo false; fi)
 #branches:
 #  only:
 #    - master