Bump chunky_png dependency ~> 1.2.7 to ~> 1.3.0

Version 1.3.0 of the ChunkyPNG gem was recently released. However,
before people who are using oily_png can upgrade, the dependency version
in oily_png's gemspec file needs to be bumped.
Merge pull request #7 from lencioni/dependency

Bump chunky_png dependency ~> 1.2.7 to ~> 1.3.0