Revert "Remove trailing commas for IE 9 compatibility"

This reverts commit 4d2e46e7f6.

Having trailing commans makes diffs easier to read as it avoids modifying a
line just to add a trailing comma if there is another item to add at the end
of a list.  There are plans to switch to ES6 notation and to translate that
to ES5 as part of the build process.  Since that translation would remove
trailing commas, the IE9 problems that originally motivated the commit
should vanish soon.

.eslintrc

@@ -4,9 +4,7 @@
         "brace-style": [2, "1tbs", { "allowSingleLine": true }],
         // We'd possibly like to remove the 'properties': 'never' one day.
         "camelcase": [2, { "properties": "never" }],
-        // IE 9 doesn't like trailing commas. TODO(emily): change this back to
-        // always-multiline once we babelify the code.
-        "comma-dangle": [2, "never"],
+        "comma-dangle": [2, "always-multiline"],
         "comma-spacing": [2, { "before": false, "after": true }],
         "constructor-super": 2,
         "curly": 2,