Fix timeout increase for building HTML files.
    
    We were increasing "ulimit -t 350" to "ulimit -t 1000" to give slower
    CPUs, such as ARM, a chance to finish within the time limit.  However,
    the source code changed to "ulimit -t 700", causing the sed expression to
    have no effect, and 700 is obviously too low, as ARM builds started
    failing again.  I have increased the limit to 1500 this time, just in case.