💎 bump version to 0.2.0

This version was motivated by feedback from DataBiosphere/toil#2064 (comment):

* use subprocess32
* use `docker.from_env(version='auto')`
* fix the way `workdir` was mapped to `/tmp` on singularity containers
* avoid shadowing of  subprocess names `def check_output` and `def check_call`
* avoid use of `/dev/null`

toil_container/VERSION

@@ -1 +1 @@
-0.1.1
+0.2.0