react loader-mixin needs to be required explicitly for npm 3+

[toolness]

This issue has been migrated from mozilla/webmaker-android#2385.

It was originally written by ashleygwilliams on Thu Jul 02 2015 and had the following description:

not urgent, but an easy fix

npm WARN peerDependencies The peer dependency react-loadermixin@* included from react-imageloader will no
npm WARN peerDependencies longer be automatically installed to fulfill the peerDependency
npm WARN peerDependencies in npm 3+. Your application will need to depend on it explicitly.