Update Installation_on_CentOS6.md
Update Installation_on_CentOS6.md

two small edits.
Line 142 added httpd-devel which added apxs.
Line 149 added a period for the ln -s
sheagcraig/sal,erikng/sal,chasetb/sal,erikng/sal,salopensource/sal,macjustice/sal,erikng/sal,salopensource/sal,salopensource/sal,sheagcraig/sal,macjustice/sal,salopensource/sal,sheagcraig/sal,chasetb/sal,macjustice/sal,sheagcraig/sal,erikng/sal,macjustice/sal,chasetb/sal,chasetb/sal