How do detect if Travis-Ci or not

I need a way to determine if the person calling the function is Travis-CI or not. If it is Travis-CI, I do not want to start a session here. Right now, I have my tests script create a file called test.txt and then look for it.

protected function __construct() {
    if ( (!session_id()) && (!file_exists('test.txt' ))) session_start();
}

However, there has to be a better way. It seems, that without the file check, if there is in fact a session made, a new one will not be created. But this is not the case. If this was the case, the before link below should have passed.

Before the addition of the "test.txt" file:
After