Enable the no-else-return ESLint rule

Using `else` after `return` is not necessary, and can often lead to unnecessarily cluttered code. By using the `no-else-return` rule in ESLint we can avoid this pattern, see http://eslint.org/docs/rules/no-else-return.

.eslintrc

@@ -45,6 +45,7 @@
     "curly": ["error", "all"],
     "eqeqeq": ["error", "always"],
     "no-caller": "error",
+    "no-else-return": "error",
     "no-eval": "error",
     "no-extend-native": "error",
     "no-extra-bind": "error",