Remove Flask version annotation so we can be installed on stretch and xenial/bionic

--- a/setup.py
+++ b/setup.py
@@ -57,7 +57,7 @@ for reqs in extras_require.values():
 setup_requires = ["Babel>=1.3", "pytest-runner>=2.6.2", "twine", "wheel"]
 
 install_requires = [
-    "Flask>=1.0.2",
+    "Flask",
     "Flask-Login>=0.4.1",
     "Flask-Mail>=0.9.1",
     "Flask-Principal>=0.4.0",
