Kill bashizm

'=' operator for test(1) is not portable.
Fix segfault on using gdk-pixbuf loader with -m option
Merge branch 'hotfix-segfault-gdk-pixbuf-with-mapfile-option' into release-1.3
Bump version
Prevent segfault with -m option in libpng loader
Merge branch 'hotfix-prevent-segfault-in-libpng-loader-with-mapfile-option' into release-1.3
Merge pull-request #15 'Kill bashism' into release-1.3
Cherry-picked edd88d0: Make img2sixel enable to load pbm files
Merge branch 'release-1.3'