tox: Update hypothesis to 1.18.0.

Features:

- Tests and find are now explicitly seeded off the global random module. This
  means that if you nest one inside the other you will now get a health check
  error. It also means that you can control global randomization by seeding
  random.
- There is a new random_module() strategy which seeds the global random module
  for you and handles things so that you don't get a health check warning if
  you use it inside your tests.
- floats() now accepts two new arguments: allow_nan and allow_infinity. These
  default to the old behaviour, but when set to False will do what the names
  suggest.

Bug fixes:

- Fix a bug where tests that used text() on Python 3.4+ would not actually be
  deterministic even when explicitly seeded or using the derandomize mode,
  because generation depended on dictionary iteration order which was affected
  by hash randomization.
- Fix a bug where with complicated strategies the timing of the initial health
  check could affect the seeding of the subsequent test, which would also
  render supposedly deterministic tests non-deterministic in some scenarios.
- In some circumstances flatmap() could get confused by two structurally
  similar things it could generate and would produce a flaky test where the
  first time it produced an error but the second time it produced the other
  value, which was not an error. The same bug was presumably also possible in
  composite().
- flatmap() and composite() initial generation should now be moderately faster.
  This will be particularly noticeable when you have many values drawn from the
  same strategy in a single run, e.g. constructs like lists(s.flatmap(f)).
  Shrinking performance may have suffered, but this didn't actually produce an
  interestingly worse result in any of the standard scenarios tested.

tox.ini

@@ -20,7 +20,7 @@ deps =
     Flask==0.10.1
     glob2==0.4.1
     httpbin==0.4.0
-    hypothesis==1.17.1
+    hypothesis==1.18.0
     itsdangerous==0.24
     Mako==1.0.3
     parse==1.6.6