Clean up color_spec.rb

Converted double quotes to single quotes and removed some unwanted
whitespace.
Add euclidean_distance_rgba

This method was recently added to ChunkyPNG[1]. From that commit
message:

  This method simply takes the Euclidean distance between the RGBA
  channels of 2 colors, which gives us a measure of how different the
  two colors are.

  Although it would be more perceptually accurate to calculate a proper
  Delta E in Lab colorspace, this method should serve many use-cases
  while avoiding the overhead of converting RGBA to Lab.

This commit bumps the ChunkyPNG dependency to 1.3.1, which contains the
new method, and adds the C implementation of this method to OilyPNG.

[1]: https://github.com/wvanbergen/chunky_png/commit/6e350b2f4a56fb3d07cf2960da51c49a389de1dc
Merge pull request #11 from lencioni/euclidean

Add euclidean_distance_rgba