[LTP] [PATCH] travis: Remove obsolete CentOS 6

[Jan Stancek]

----- Original Message -----
> We are planning to remove the support of old distros(e.g. CentOS 6).
> 
> Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>

Acked-by: Jan Stancek <jstancek@redhat.com>