Fixes #1600 - Invalid path returned by sprite_file

When specifiying a generic_images_dir sprite_file would return the wrong
path to the original image because this was relative to the generic_
images_path instead of the images_path.
Bump sass requirement.
Update caniuse data.
Update CHANGELOG
Merge remote-tracking branch 'eddie/bug/fix_sprite_file_path'
Actual dates.
Generate a ruby file with the release version at build time instead of reading it at runtime.
Generate a ruby file with the release version at build time instead of reading it at runtime.