README.linux.md: SCL repository must be installed before devtoolset-6 packages
README.linux.md: SCL repository must be installed before devtoolset-6 packages

Signed-off-by: David Sharp <2664301c0a809f94c834d0a9c6cef510e103d89b@pivotal.io>
ashwinstar/gpdb,yuanzhao/gpdb,xinzweb/gpdb,Chibin/gpdb,Chibin/gpdb,adam8157/gpdb,ashwinstar/gpdb,Chibin/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,Chibin/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,Chibin/gpdb,greenplum-db/gpdb,edespino/gpdb,ashwinstar/gpdb,50wu/gpdb,edespino/gpdb,edespino/gpdb,greenplum-db/gpdb,xinzweb/gpdb,ashwinstar/gpdb,50wu/gpdb,edespino/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,xinzweb/gpdb,xinzweb/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,adam8157/gpdb,Chibin/gpdb,lisakowen/gpdb,50wu/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,50wu/gpdb,edespino/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,edespino/gpdb,Chibin/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,Chibin/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,edespino/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,edespino/gpdb,yuanzhao/gpdb,Chibin/gpdb,ashwinstar/gpdb,xinzweb/gpdb,yuanzhao/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,edespino/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,Chibin/gpdb,edespino/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,50wu/gpdb,lisakowen/gpdb