Preparing for 1.7.4

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

sample/doc/payments/OrderAuthorize.html

@@ -29,4 +29,4 @@ We will get a new authorization object back.</p></div></div><div class="code"><d
     <span class="hljs-keyword">return</span> <span class="hljs-variable">$result</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>