Update PATH for the right PostgreSQL version's psql tools.

There's a potential for conflict when using different versions
of the command line tools (installed in /usr/bin, overwriting each
other currently) are used with a different version of the server.

As we don't have full control about which version of the tools are
installed in /usr/bin, this change automatically prepends the right
version to the PATH.
Merge pull request #181 from travis-ci/mm-postgresql-addon-sets-path

Update PATH for the right PostgreSQL version's psql tools