Remove trailing slash in the api url (unit-finance#116)

The [Unit docs](https://docs.unit.co/) include the trailing slash, so
the sdk should handle a base url both with the trailing slash (to be
consistent with the docs) and without it (for backwards compatability)

Author: jhhayashi

unit/api/base_resource.py

@@ -6,7 +6,7 @@
 
 class BaseResource(object):
     def __init__(self, api_url, token):
-        self.api_url = api_url
+        self.api_url = api_url.rstrip("/")
         self.token = token
         self.headers = {
             "content-type": "application/vnd.api+json",