Replace accent character using sub instead of tr

When replace accent character in PinYin with normal English character,
do not use "tr", but use "sub" instead. Because for ruby 1.8, a accent
character is treat as two or more characters and when replace using
"tr", it will be replaced with two or more normal characters.
E.g.

    Input: jiǎ
    Replace with tr:  jiaa3
    Replace with sub: jia3

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>