Log Message:
Updated py-virtualenv to 15.0.3.

15.0.3 (2016-08-05)

    Test for given python path actually being an executable file, #939
    Only search for copy actual existing Tcl/Tk directories (PR #937)
    Generically search for correct Tcl/Tk version (PR #926, PR #933)
    Upgrade setuptools to 22.0.5