Upgrade depreciated transitions addon to react-transition-group.

v1 of `react-transition-group` is a drop-in replacement.

package.json

@@ -129,7 +129,6 @@
     "prismjs": "^1.5.1",
     "prop-types": "^15.5.10",
     "react": "^15.1.0",
-    "react-addons-css-transition-group": "^15.3.1",
     "react-autosuggest": "^7.0.1",
     "react-datetime": "^2.6.0",
     "react-dom": "^15.1.0",
@@ -149,6 +148,7 @@
     "react-textarea-autosize": "^4.3.2",
     "react-toolbox": "^1.2.1",
     "react-topbar-progress-indicator": "^1.0.0",
+    "react-transition-group": "1.x",
     "react-waypoint": "^3.1.3",
     "redux": "^3.3.1",
     "redux-notifications": "^2.1.1",