Take advantage of PHP 5.4+ short echo (#2585)

* Take advantage of PHP 5.4+ short echo

https://php.net/migration54.new-features thanks to
https://github.com/FreshRSS/FreshRSS/pull/2495

Use `<?=  ?>` instead of `<?php echo; ?>`
10kB of code saved :-)

Done with regular expression:
```
<\?php echo (.+?);? *\?>
<?= \1 ?>
```

* Try Travis fix

https://github.com/squizlabs/PHP_CodeSniffer/issues/2045#issuecomment-395238272

phpcs.xml

@@ -39,7 +39,9 @@
 	<!-- Do not add a space before a comma -->
 	<!-- Add a space after a comma -->
 	<rule ref="Generic.Functions.FunctionCallArgumentSpacing"/>
-	<rule ref="Generic.PHP.DisallowShortOpenTag" />
+	<rule ref="Generic.PHP.DisallowShortOpenTag">
+		<exclude name="Generic.PHP.DisallowShortOpenTag.EchoFound"/>
+	</rule>
 	<rule ref="Generic.PHP.DeprecatedFunctions" />
 	<!-- Use UPPERCARE for constants -->
 	<rule ref="Generic.NamingConventions.UpperCaseConstantName"/>