Remove chunky_png from Gemfile

The oily_png gem, which is a Ruby C extension to speed up the pure Ruby
ChunkyPNG library, has chunky_png as a dependency. So we may as well
remove chunky_png from our Gemfile.

There is a newer version of chunky_png out (1.3.0), but oily_png hasn't
been updated yet, so we can't upgrade until that project is ready.

Change-Id: I4c0dbbf4ece857de5ccd599f0f6eeeee2b31c0f6
Update aws-sdk gem 1.32.0 -> 1.33.0

Changes can be found at:

  https://github.com/aws/aws-sdk-ruby/compare/a609d71519e0a449bf0685fd89b88ca8427dd767...22278262a464f512137e0ff132f2a84ea86faa15

Change-Id: Ia4ac9615ca1d1fab6ed7f877e0b94a87ec46c49e