Prevent builds from failing the deploy [ci skip]

Every job runs the build stage, which is fine, since we need the different `wheel` packages, but they failed, since the files were already present on PyPI

.travis.yml

@@ -31,6 +31,7 @@ deploy:
   password:
     secure: "VA0MLSrwIW/T2KjMwjLZCzrLHw8pJT6tAvb48t7qpBdm8x192hax61pz1TaBZoJvlzyBPFKvluftuclTc7yEFwzXe7Gjqgd/ODKZl/wXDr36hQ7BBOLPZujdwmWLvTzMh3eJZlvkgcLCzrvK3j2oW8cM/+FZeVi/5/FhVuJ4ofs="
   distributions: sdist bdist_wheel
+  skip_existing: true
   on:
     branch: master
     tags: true