tox: Update pytest-qt to 1.4.0.

Upstream changelog:

- Messages sent by qDebug, qWarning, qCritical are captured and displayed when
  tests fail, similar to pytest-catchlog. Also, tests can be configured to
  automatically fail if an unexpected message is generated. (See docs).
- New method waitSignals: will block untill all signals given are triggered, see
  docs (thanks @The-Compiler for idea and complete PR).
- New parameter raising to waitSignals and waitSignals: when True (defaults to
  False) will raise a qtbot.SignalTimeoutError exception when timeout is reached,
  see docs (thanks again to @The-Compiler for idea and complete PR).
- pytest-qt now requires pytest version >= 2.7.

Internal changes to improve memory management

- QApplication.exit() is no longer called at the end of the test session and
  the QApplication instance is not garbage collected anymore;
- QtBot no longer receives a QApplication as a parameter in the constructor,
  always referencing QApplication.instance() now; this avoids keeping an extra
  reference in the qtbot instances.
- deleteLater is called on widgets added in QtBot.addWidget at the end of each
  test;
- QApplication.processEvents() is called at the end of each test to make sure
  widgets are cleaned up;

tox.ini

@@ -23,7 +23,7 @@ deps =
     py==1.4.28
     pytest==2.7.1
     pytest-capturelog==0.7
-    pytest-qt==1.3.0
+    pytest-qt==1.4.0
     pytest-mock==0.6.0
     pytest-html==1.3.1
 # We don't use {[testenv:mkvenv]commands} here because that seems to be broken