Add vpn app for NetBox 3.7 compatibility

[markkuleinio]

Fixes: #595

Adds support for vpn app.

Before this patch:

>>> nb.version
'3.7'
>>> list(nb.vpn.tunnels.filter())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Api' object has no attribute 'vpn'
>>>

After this patch:

>>> list(nb.vpn.tunnels.filter())
[Tunnel1]
>>>

[markkuleinio]

Now that NetBox 3.7.0 is out, any comments from @arthanson or @abhi1693 ?

[kuhball]

Hi, would it be possible to create a new release containing this merged PR? :)