.travis.yml: Exclude openrd platforms

The openrd platforms are currently orphaned, and are constantly on-edge
or overflowing their binary limit.  Exclude them from travis for now.

Cc: Vagrant Cascadian <vagrant@debian.org>
Cc: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>