vagrant: update to Python 3.6.5 (for binary builds)

Vagrantfile

@@ -54,7 +54,7 @@ def install_pythons(boxname)
     . ~/.bash_profile
     pyenv install 3.5.0  # tests
     pyenv install 3.6.0  # tests
-    pyenv install 3.6.2  # binary build, use latest 3.6.x release
+    pyenv install 3.6.5  # binary build, use latest 3.6.x release
     pyenv rehash
   EOF
 end
@@ -72,8 +72,8 @@ def build_pyenv_venv(boxname)
     . ~/.bash_profile
     cd /vagrant/borg
     # use the latest 3.6 release
-    pyenv global 3.6.2
-    pyenv virtualenv 3.6.2 borg-env
+    pyenv global 3.6.5
+    pyenv virtualenv 3.6.5 borg-env
     ln -s ~/.pyenv/versions/borg-env .
   EOF
 end