Travis-CI: Drop support

Travis-CI is changing their support for FOSS (understandably) to have a
limited per-month number of build minutes.  Unfortunately for us, the
matrix of jobs we run will exhaust that very quickly.  Remove the yml
file.  Thanks for all the builds, Travis-CI!

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>