Don't install traitsui from source after 7.1.0 release

[rahulporuri]

Once traitsui 7.1.0 is released (currently underway), it would be nice to stop installing traitsui from source - and instead use the released version of traitsui.

See https://github.com/enthought/pyface/blob/master/ci-src-requirements.txt and

pyface/etstool.py

Lines 94 to 99 in 3acd644

	dependencies = {
	"traits" + TRAITS_VERSION_REQUIRES,
	"numpy",
	"pygments",
	"coverage",
	}