Make sure the TraitsUI control is destroyed then the Enaml object is.

I wasn't able to create a minimal failing example, but this caused a
segfault in a UI in a customer project.
PEP8
Revised TraitsView to un-parent control before disposing.
Update dev_requirements.txt
Merge pull request #19 from enthought/fix-traits-view-destroy

Make sure the TraitsUI control is destroyed then the Enaml object is.