Merge branch 'develop' into nj/ui-changes

.travis.yml

package.json

src/components/sprite-selector/sprite-selector.jsx

src/components/stage-selector/stage-selector.jsx

src/containers/costume-tab.jsx

src/containers/sound-tab.jsx

src/containers/stage-selector.jsx

src/containers/target-pane.jsx

src/lib/file-uploader.js

src/lib/gif-decoder.js

test/fixtures/paddleball.gif

test/integration/backdrops.test.js

test/integration/costumes.test.js

test/integration/sprites.test.js