Allure report does not show diff screenshot

[ashiots2]

Allure report shows the diff screenshot as corrupted; since the fs.writeFile() call is not finished when allure.addAttachment is called.

I replaced the fs.writeFile() call with fs.writeFileSync() in #51

Also covered some warnings the IDE highlighted.

Again, looking forward to your review :)

[puneet0191]

Fixed thanks