differentiate SPM5 and SPM8: SPM8 lacks spm_vol_check.m

Matthew,

In this pull request I am suggesting some changes that fix the issue of the missing spm_vol_check.m in all recent versions of SPM8.

See this thread on the MarsBaR mailing list:
http://sourceforge.net/mailarchive/forum.php?thread_name=25EC1341450371479BBC0175CB93DBA53EAD4814%40EXMB2501.ds.umcutrecht.nl&forum_name=marsbar-users

Best regards,
-- 
Dimitri