vagrant: add Python 3.6.0

Vagrantfile

@@ -223,6 +223,7 @@ def install_pythons(boxname)
     . ~/.bash_profile
     pyenv install 3.4.0  # tests
     pyenv install 3.5.0  # tests
+    pyenv install 3.6.0  # tests
     pyenv install 3.5.2  # binary build, use latest 3.5.x release
     pyenv rehash
   EOF
@@ -315,7 +316,7 @@ def run_tests(boxname)
     . ../borg-env/bin/activate
     if which pyenv 2> /dev/null; then
       # for testing, use the earliest point releases of the supported python versions:
-      pyenv global 3.4.0 3.5.0
+      pyenv global 3.4.0 3.5.0 3.6.0
     fi
     # otherwise: just use the system python
     if which fakeroot 2> /dev/null; then