make redoc version configurable (#137)

Author: alorenzo175

sfa_api/config.py

@@ -9,7 +9,7 @@
 
 class Config(object):
     API_VERSION = __version__
-    REDOC_VERSION = 'next'
+    REDOC_VERSION = os.getenv('REDOC_VERSION', 'next')
     AUTH0_BASE_URL = 'https://solarforecastarbiter.auth0.com'
     AUTH0_AUDIENCE = 'https://api.solarforecastarbiter.org'
     JWT_KEY = requests.get(