fix: Use weak etags instead of etag (#9)

We are not conforming to strong etags anyway and they are dropped
by cloudlare and nginx when response is compressed as it is not
hash of final output response

.travis.yml

@@ -1,9 +1,6 @@
 language: python
 python:
-  - '2.7'
-  - '3.4'
-  - '3.5'
-  - 'pypy'
+  - '3.7'
 install:
   - pip install -r requirements.txt
   - pip install coveralls coverage

flask_rest_jsonapi/resource.py

@@ -109,7 +109,7 @@ def dispatch_request(self, *args, **kwargs):
 
         # ETag Handling
         if current_app.config.get('ETAG') == True:
-            etag = hashlib.sha1(resp.get_data()).hexdigest()
+            etag = f'W/"{hashlib.sha1(resp.get_data()).hexdigest()}"'
             resp.headers['ETag'] = etag
 
             if_match = request.headers.get('If-Match')