Upgrade devtoolset version for el6

Currently for building RHEL6 based distributions, kudu uses devtoolset3.
However devtoolset3 is EOL as of Nov 2016 and it's repo location has been removed from the official software collections repository. See related KUDU-3159.

So upgrade devtoolset to a newer version for building kudu on RHEL6 based distributions.
Apart from the fact that devtoolset3 is EOL and not available officially, newer version of devtoolset will also lead to generation of better code.
See comment from tlipcon https://gerrit.cloudera.org/c/16114/#message-86ea8228_e84eeb60