Modify .gitlab-ci.yml so that every CI pipeline also builds and tests
BIND on CentOS versions 6 and 7.  Use --disable-warn-error on CentOS 6
since it uses GCC 4.4.7 which suffers from bugs causing bogus warnings
to be generated, e.g.:

    sigs_test.c: In function 'compare_tuples':
    sigs_test.c:75: warning: declaration of 'index' shadows a global declaration
    /usr/include/string.h:489: warning: shadowed declaration is here
    sigs_test.c: In function 'updatesigs_test':
    sigs_test.c:193: warning: declaration of 'index' shadows a global declaration
    /usr/include/string.h:489: warning: shadowed declaration is here