tox: Update hypothesis to 1.11.3

- Better argument validation for datetimes() strategy - previously setting
  max_year < datetime.MIN_YEAR or min_year > datetime.MAX_YEAR would not have
  raised an InvalidArgument error and instead would have behaved confusingly.
- Compatibility with being run on pytest < 2.7 (achieved by disabling the
  plugin).

tox.ini

@@ -21,7 +21,7 @@ deps =
     pytest-qt==1.7.0
     pytest-mock==0.7.0
     pytest-html==1.6
-    hypothesis==1.11.2
+    hypothesis==1.11.3
     coverage==4.0.0
     pytest-cov==2.1.0
     beautifulsoup4==4.4.0