Fix syntax of bullet image reference in styles.css

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
Use get_custom_data() method in 25_STABLE branch (deprecated in master)
Use get_url() method in 25_STABLE branch

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
Update version.php for Moodle 2.5.x release

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
Increment build number for 2.5.1 release

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>