Add flask to dependencies list and requirements.txt

requirements.txt

+flask
 invenio-access>=1.0.0b1
 invenio-app>=1.0.0b1,<1.1.0
 invenio-base>=1.0.0a15,<1.1.0

setup.py

@@ -52,6 +52,7 @@ install_requires = [
     'invenio-oauthclient>=1.0.0b5',
     'invenio-search[elasticsearch5]>=1.0.0a10,<1.1.0',
     'redis>=2.10.0',
+    'flask',
 ]
 
 packages = find_packages()