Remove executive rights from assets

[strzibny]

New version of sprite-factory have an executive bit set for some assets. This is what shows gem-compare:

$ gem compare sprite-factory 1.6.2 1.7 -kb
Fetching: sprite-factory-1.7.gem (100%)
Compared versions: ["1.6.2", "1.7"]
  DIFFERENT files:
    1.6.2->1.7:
      * Added:
            test/images/glob/
            test/images/names/
            test/images/glob/excluded1.png
              (!) Unexpected permissions: 100775
              (!) File is executable
            test/images/glob/excluded2.png
              (!) Unexpected permissions: 100775
              (!) File is executable
            test/images/glob/included1.png
              (!) Unexpected permissions: 100775
              (!) File is executable
            test/images/glob/included2.png
              (!) Unexpected permissions: 100775
              (!) File is executable
            test/images/glob/included3.png
              (!) Unexpected permissions: 100775
              (!) File is executable
            test/images/names/Ends With Bang!.png
              (!) Unexpected permissions: 100775
              (!) File is executable
            test/images/names/Has & Ampersand.png
              (!) Unexpected permissions: 100775
              (!) File is executable
            test/images/names/Odd.Period.png
              (!) Unexpected permissions: 100775
              (!) File is executable
            test/images/reference/glob.css
              (!) Unexpected permissions: 100664
            test/images/reference/glob.png
              (!) Unexpected permissions: 100664
            test/images/reference/sanitized.css
              (!) Unexpected permissions: 100664
            test/images/reference/sanitized.custom.css
              (!) Unexpected permissions: 100664
            test/images/reference/sanitized.custom.png
              (!) Unexpected permissions: 100664
            test/images/reference/sanitized.png
              (!) Unexpected permissions: 100664
    1.6.2->1.7:
      * Updated
            rmagick from: ["= 2.13.2"] to: ["= 2.13.4"]

This PR fixes the issue.