Fix Travis build
Fix Travis build

The Travis build was broken by optee_test commit 0e00914b378d ("Make
cross compile variables configurable via the environment"), because the
CC environment variable is set to the host compiler during Travis
builds. Simply unset this variable in the before_script step to avoid
any side-effect.

Signed-off-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>
Reviewed-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>

matt2048/optee_os,matt2048/optee_os,pascal-brand-st-dev/optee_os,cedric-chaumont-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,Microsoft/optee_os,cedric-chaumont-st-dev/optee_os,Microsoft/optee_os,matt2048/optee_os,cedric-chaumont-st-dev/optee_os,cedric-chaumont-st-dev/optee_os,pascal-brand-st-dev/optee_os,Microsoft/optee_os,matt2048/optee_os,Microsoft/optee_os,pascal-brand-st-dev/optee_os,Microsoft/optee_os