FIX: New versions of FreeImage report software as "I", not "ImageMagick ..."

This should close both #820 and #730.

It was a bit interesting. It appears some versions of FreeImage - older ones, apparently - reported `('EXIF_MAIN', 'Software')` as `"ImageMagick [date] [version number] [other stuff]". The old version available through my Enterprise Linux derivative's package manager did this - though it was broken in other ways for being so out of date. Thus, we just checked for the string to start with `"ImageMagick"`.

After removing that version of FreeImage, I went and compiled an entirely fresh install from source. And now I can reproduce the errors reported in #820 and #730. New versions of FreeImage are reporting `('EXIF_MAIN', 'Software')` as the single character `'I'`.

The fix is simple and backwards compatible, just change the test to `.startswith('I')`, and should be credited to @rovitotv.