[AIRFLOW-2391] Fix to Flask 0.12.2

Flask 0.12.3 has issues with Airflow and needs to
be fixed.
Therefore lock the version to 0.12.2.

Closes #3277 from Fokko/fd-fix-master-ci

(cherry picked from commit 3368f42)
Signed-off-by: Fokko Driesprong <fokkodriesprong@godatadriven.com>

setup.py

@@ -229,7 +229,7 @@ def do_setup():
             'configparser>=3.5.0, <3.6.0',
             'croniter>=0.3.17, <0.4',
             'dill>=0.2.2, <0.3',
-            'flask>=0.12, <0.13',
+            'flask==0.12.2',
             'flask-appbuilder>=1.9.6, <2.0.0',
             'flask-admin==1.4.1',
             'flask-caching>=1.3.3, <1.4.0',