travis: use correct form for array options

I'd like to eventually drop support for the confusing "an array of
a single empty string is meant to be interpreted as an empty array", so
let's start by not using it anymore.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
1 file changed