Uninterruptible infinite loop on editing out-of-range value in RangeEditor

When entering an out of range value in an int-based RangeEditor, I'm getting an unresponsive application and the following output generated in a loop on the console:
```
2018-07-05 13:51:27.769 Python[37303:887982] -[NSApplication runModalSession:] may not be invoked inside of transaction begin/commit pair, or inside of transaction commit (usually this means it was invoked inside of a view's -drawRect: method.) The modal dialog has been suppressed to avoid deadlock. (
0 AppKit 0x00007fff30a93a59 -[NSApplication runModalSession:] + 187
1 QtGui 0x00000001056cbadd _ZN19QEventDispatcherMac13processEventsE6QFlagsIN10QEventLoop17ProcessEventsFlagEE + 493
2 QtCore 0x0000000103f7dcb2 _ZN10QEventLoop4execE6QFlagsINS_17ProcessEventsFlagEE + 498
)
```

To reproduce:

1. Execute `python example.py` where `example.py` is the script shown below.
2. Enter 101 in the text box and press ``

Versions: Python 2.7.13, Traits UI 6.0.0, Traits 4.6.0, macOS 10.13.5, PyQt 4.12.

Script:
```python
from traits.api import HasStrictTraits, Range
from traitsui.api import Item, View

class RangeView(HasStrictTraits):
input = Range(low=0, high=100)

traits_view = View(Item('input'))

if __name__ == '__main__':
view = RangeView()
view.configure_traits()
```