[AIRFLOW-1915] Relax flask-wtf dependency specification

Closes #2876 from wrp/flask-wtf

setup.py

@@ -208,7 +208,7 @@ def do_setup():
             'flask-cache>=0.13.1, <0.14',
             'flask-login==0.2.11',
             'flask-swagger==0.2.13',
-            'flask-wtf==0.14',
+            'flask-wtf>=0.14, <0.15',
             'funcsigs==1.0.0',
             'future>=0.16.0, <0.17',
             'gitpython>=2.0.2',