Log Message:
Update ruby-chunky_png to 1.3.7.

=== 1.3.7 - 2016-08-31

- Performance improvement for Color.euclidean_distance_rgba.
- Bugfix in decoding transparent pixels when decoding multiple images in a row.

=== 1.3.6 - 2016-06-19

- Allow reading images from streams that have trailing data after the IEND chunk.
- Add compatibility for Ruby 2.3's frozen string literals.
- Documentation updates and small cleanups.