Do not call non-static method statically (PEAR::raiseError)

Calling PEAR::raiseError() results in a deprecation warning (and thus CI
error) with PHP5.6.  An alternative fix would be to use a
PEAR_Exception, but I wasn't quite sure what to do with
$this->_pearErrorMode (Nominally, I'd have used PEAR::setErrorHandling,
but that ends up emitting the same deprecation warning, and simply
changing to an exception would have required more invasive changes to
all callers.

Signed-off-by: Nishanth Aravamudan <nish.aravamudan@canonical.com>

Cache/Lite.php

@@ -555,7 +555,8 @@ function lastModified()
     function raiseError($msg, $code)
     {
         include_once('PEAR.php');
-        return PEAR::raiseError($msg, $code, $this->_pearErrorMode);
+        $pear = new PEAR();
+        return $pear->raiseError($msg, $code, $this->_pearErrorMode);
     }
 
     /**