readme: Remove references to travis-ci
readme: Remove references to travis-ci

Travis-ci is not used anymore. Remove references to it.

Signed-off-by: Jere Leppänen <14957520de8d9815e0c353a07fe23c737f0171e9@nokia.com>
Reviewed-by: Matias Elo <62402263e8617147f0e4dd5b7f6f8ec67f2b3d2c@nokia.com>

OpenFastPath/ofp,OpenFastPath/ofp,OpenFastPath/ofp,OpenFastPath/ofp