[README] Clarify relaitonship b/w checks in travis.sh and Prerelease Test.
[README] Clarify relaitonship b/w checks in travis.sh and Prerelease Test.

ros-industrial/industrial_ci,geoffreychiou/industrial_ci,ubi-agni/industrial_ci,ros-industrial/industrial_ci,tecnalia-advancedmanufacturing-robotics/industrial_ci,ubi-agni/industrial_ci,tecnalia-advancedmanufacturing-robotics/industrial_ci,ros-industrial/industrial_ci,geoffreychiou/industrial_ci,tecnalia-advancedmanufacturing-robotics/industrial_ci