Update composer.json

Merge pull request #49 from devduttabain/patch-1

verifying checksum only with posted inputs.

Merge pull request #42 from InfoSecRavindra/master

Fixed #40

update syntax

update composer

get response as array instead of object

changes commited by @hardevine 

pull request : #30

doc update and merge

refund docs

Update README.md

Merge pull request #19 from plakhin/master

updates payment GW URLs