Maintenance: Use __DIR__ instead of dirname( __FILE__ )

Maintenance: Use __DIR__ instead of dirname( __FILE__ )

dirname( __FILE__ ) was used because it worked in PHP 5.2, allowing
the version check to function correctly. However, Maintenance.php
now uses the short array syntax, which is a parse error before 5.4:

> Parse error: syntax error, unexpected '[' in maintenance/Maintenance.php
> on line 66

Also, do the same in install.php where it includes Maintenance.php.

Change-Id: I3373a02c684bb6cbde83d7bedc204818f3394602