Don't be strict about the React dependencies

Because these are peer dependencies, we should be lenient about the required version of React required to use the components.

I didn't use the caret because the x's work and it's obvious what they mean, unlike ^ and ~.

package.json

@@ -15,8 +15,8 @@
     "url": "git+ssh://git@github.com/LLK/scratch-gui.git"
   },
   "peerDependencies": {
-    "react": "15.3.1",
-    "react-dom": "15.3.1"
+    "react": "15.x.x",
+    "react-dom": "15.x.x"
   },
   "devDependencies": {
     "babel-core": "6.14.0",