Update xidmodifications.txt for Unicode 6.3?

http://www.unicode.org/Public/security/latest/xidmodifications.txt is
still the 6.2 version and has not been updated to include changes in 6.3

There are at least two such changes that will affect xidmod: Firstly,
U+180E MONGOLIAN VOWEL SEPARATOR should change from "restricted ;
not-xid" to "restricted ; default-ignorable". This may not make much
practical difference, but more seriously the new U+061C ARABIC LETTER
MARK needs to be added to "restricted ; default-ignorable".

-- 
Simon Montagu
Mozilla internationalization
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