README.md: change python-dev to python-devel for CentOS.
README.md: change python-dev to python-devel for CentOS.

Thanks to Gao Yongwei <itxx00@gmail.com> for the report.

Signed-off-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

jbaber/bup,pombredanne/bup,mhoeher/bup,mhoeher/bup,tjanez/bup,ToxicFrog/bup,ToxicFrog/bup,gevaerts/bup,pombredanne/bup,tjanez/bup,mhoeher/bup,tjanez/bup,jbaber/bup,tjanez/bup,pombredanne/bup,gevaerts/bup,mhoeher/bup,pombredanne/bup,gevaerts/bup,mhoeher/bup,jbaber/bup,jbaber/bup,jbaber/bup,gevaerts/bup,ToxicFrog/bup,ToxicFrog/bup,pombredanne/bup,pombredanne/bup,tjanez/bup,tjanez/bup,jbaber/bup,ToxicFrog/bup,gevaerts/bup,mhoeher/bup