Change RGB to SRGB for newer versions of ImageMagick

Since ImageMagick 6.7.5-5 (released early 2012), the colorspace "sRGB" has to be used instead of "RGB". (https://www.imagemagick.org/discourse-server/viewtopic.php?t=20501). If RGB is used with newer versions, images appear too dark (see https://collectiveaccess.org/support/index.php?p=/discussion/299423/providence-v1-3-now-produces-images-either-too-light-or-too-dark)

I changed

array_unshift($va_ops['convert'], '-colorspace RGB');
to

array_unshift($va_ops['convert'], '-colorspace sRGB');
on https://github.com/collectiveaccess/providence/blob/master/app/lib/core/Plugins/Media/ImageMagick.php#L1273

It probably makes sense to make this configurable somewhere as long as there are still systems with older versions of ImageMagick in use.