fixes #40 (The expression of an export assignment must be...)

[huben92]

• fixes The expression of an export assignment must be an identifier or qualified name in an ambient context. #40
• Comply to typescript version 2.6 and above
• Tested in typescript version 3.4, typescript above 3.4 may work but not guarantee.
• Typescript reference: https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#arbitrary-expressions-are-forbidden-in-export-assignments-in-ambient-contexts

[stanleynguyen]

Awesome! Thanks for the contrib @huben92 . Quick question, would this work without changing the declaration for sub-packages (i.e. invoices https://github.com/xendit/xendit-node/blob/master/src/invoice/invoice.d.ts). Have you tried this with your local application that's dependent on xendit-node? (You can modify the file directly inside your node_modules)

[huben92]

Yes, this work without changing sub-package,
Yes, i tested on my local application inside node_modules, try to build with typescript 3.4 and it work fine.