Remove patching class ExpensiMark with an object #41

Currently, we need to patch expensify-common/lib/ExpensiMark.js to convert class ExpensiMark into a single object because Hermes does not support ES Classes (facebook/hermes#685 (comment)).

In particular, we had to make the following changes:

1. Remove class keyword:

-export default class ExpensiMark {
+export const ExpensiMark = {

2. Rename constructor to initializer:

-    constructor() {
+    initializer() {

3. Add commas after method definitions:

-    }
+    },

It would be better to use a Babel plugin to automatically convert class ExpensiMark into something that Hermes can deal with.