Fix test errors in Loader_test.php and URI_test.php

Change exceptions from Exception to RuntimeException since PHPUnit 3.6
doesn't like you to expect generic exceptions. The error it gives is:

InvalidArgumentException: You must not expect the generic exception class
travis-ci.org/#!/tiyowan/CodeIgniter/builds/832518

This issue addressed by using exceptions that are more specific.
diff --git a/tests/lib/common.php b/tests/lib/common.php
index 6d29eb0..4a83258 100644
--- a/tests/lib/common.php
+++ b/tests/lib/common.php
@@ -87,17 +87,17 @@
 
 function show_error($message, $status_code = 500, $heading = 'An Error Was Encountered')
 {
-	throw new Exception('CI Error: '.$message);
+	throw new RuntimeException('CI Error: '.$message);
 }
 
 function show_404($page = '', $log_error = TRUE)
 {
-	throw new Exception('CI Error: 404');
+	throw new RuntimeException('CI Error: 404');
 }
 
 function _exception_handler($severity, $message, $filepath, $line)
 {
-	throw new Exception('CI Exception: '.$message.' | '.$filepath.' | '.$line);
+	throw new RuntimeException('CI Exception: '.$message.' | '.$filepath.' | '.$line);
 }
 
 
@@ -129,4 +129,4 @@
 	return TRUE;
 }
 
-// EOF
\ No newline at end of file
+// EOF