Update medium-zoom version to 0.4.0 (IE10/11 compatibility)

[jhildenbiddle]

Docsify's Zoom Image plugin is based on medium-zoom v0.3.0. This version throws an error in IE, which in turn prevents other JavaScript from executing. This effectively breaks docsify-powered sites in IE.

I submitted a PR that addresses the issue. The PR has been merged and made available in v0.4.0. If docsify's version is updated to 0.4.0, these issues should be resolved for docsify sites using the zoom image plugin.

Thanks!