Only choose from what's offered

Typing "1.5.8" will give you Django 1.6.x, anyway, presumably because "1.5.8" isn't any of the listed choices, so it's falling back to the default. (It'd be much nicer if the installer would abort on invalid input, but apparently, it doesn't.) Typing just "1.5" will give us Django 1.5.x, thereby working around the "atomic transactions" problem mentioned in 2243f26bd2397abc9282f9c64957eb3bdc14656f