Bump twisted dependency to 17.1.0

See matrix-org/synapse#3741 for some background here;
essentially we are now incompatible with Twisted 16.x.

debian/changelog

@@ -1,3 +1,9 @@
+matrix-synapse (0.33.3-2) UNRELEASED; urgency=medium
+
+  * We now require python-twisted 17.1.0 or later
+
+ -- Richard van der Hoff <richard@matrix.org>  Thu, 23 Aug 2018 18:28:58 +0100
+
 matrix-synapse (0.33.3-1) jessie; urgency=medium
 
   * New upstream version 0.33.3

debian/control

@@ -27,7 +27,7 @@ Build-Depends:
  python-signedjson (>= 1.0.0),
  python-sortedcontainers,
  python-syutil (>= 0.0.7),
- python-twisted (>= 16.0.0),
+ python-twisted (>= 17.1.0),
  python-unpaddedbase64 (>= 1.0.1),
  python-yaml,
  python-phonenumbers (>= 8.2.0),
@@ -45,7 +45,7 @@ Depends:
  adduser,
  debconf,
  lsb-base (>= 3.0-6),
- python-twisted (>= 16.0.0),
+ python-twisted (>= 17.1.0),
  python-bcrypt,
  python-canonicaljson (>=1.1.3),
 # explicitly depend on attr because 0.31.0 forgot it in python_dependencies