ROS_DISTRO set to hydro after indigo install

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In Linux, my .bashrc had only one line pertaining to ROS:

source /opt/ros/indigo/setup.bash

Yet I was getting the following error message upon opening a new shell:

ROS_DISTRO was set to 'hydro' before. Please make sure that the environment does not mix paths from different distributions.

It turns out that due to some shell caching or something, ROS_DISTRO was stuck on 'hydro' even after removing the source line from the .bashrc. This problem was solved by simply logging out and back in.