build: Remove unused USE_SYSTEM_BOOST flag from Travis build
build: Remove unused USE_SYSTEM_BOOST flag from Travis build

NVlabs/firepony,broadinstitute/firepony,broadinstitute/firepony,broadinstitute/firepony,broadinstitute/firepony,NVlabs/firepony,NVlabs/firepony,NVlabs/firepony