(fix) use imagemin in prod only

Before, imagemin was always ran during build. Now, imagemin is only
ran in prod stage.
(fix) use build directory in wiredep replaces

Before, wiredep always injected styles and scripts into css/ and js/.
Now, injects styles and scripts into directories listed in
build.config.js.
(chore) update generated dependencies
(chore) bump version to 0.2.20