Preparing for 1.7.4

- Updated Changelog
- Updated PayPalConstants.php with latest version
- Added new line at the end of file

sample/doc/payments/ExecutePayment.html

@@ -54,4 +54,4 @@ Here is the example on how we changed the shipping to $1 more than before.</p></
     <span class="hljs-keyword">return</span> <span class="hljs-variable">$payment</span>;
 } <span class="hljs-keyword">else</span> {</div></div></div><div class="segment"><div class="comments "><div class="wrapper"><p>NOTE: PLEASE DO NOT USE RESULTPRINTER CLASS IN YOUR ORIGINAL CODE. FOR SAMPLE ONLY</p></div></div><div class="code"><div class="wrapper">    ResultPrinter::printResult(<span class="hljs-string">"User Cancelled the Approval"</span>, <span class="hljs-keyword">null</span>);
     <span class="hljs-keyword">exit</span>;
-}</div></div></div></div></body></html>
+}</div></div></div></div></body></html>