Update Python version mentioned in CONTRIBUTING.md
@@ -15,7 +15,7 @@ To develop & test repo2docker locally, you need:
1. Familiarity with using a command line terminal
2. A computer running macOS / Linux
3. Some knowledge of git
-4. At least python 3.4
+4. At least python 3.6
5. Your favorite text editor
6. A recent version of [Docker Community Edition](https://www.docker.com/community-edition)
