Fix broken HTML in limitCompute exercise: removes duplicate content

[ento]

This patch fixes the same problem as #80, but also goes on to remove duplicate HTML at the end of the file.

Without this fix, rake exercise:all throws errors like:

ERROR: A Problem in /public/khan-exercises/exercises/limitCompute.html doesn not have a div id
ERROR: Not all the exercise and problems were created
ERROR: Please fix problem div tags and run task again

Before this fix, limitCompute.html has duplicate IDs:

$ grep id= limitCompute.html | cut -d '"' -f 2 | sort | uniq -c
   2 Quadratic-over-linear
   2 Ratio-of-two-quadratics-one-linear-term-cancels
   2 derivative-of-x2
   2 derivative-of-x3
   2 derivative-of-x4
   2 needs-conjugate
   2 sine-x-over-x

After applying this fix:

$ grep id= limitCompute.html | cut -d '"' -f 2 | sort | uniq -c
   1 Quadratic-over-linear
   1 Ratio-of-two-quadratics-one-linear-term-cancels
   1 derivative-of-x2
   1 derivative-of-x3
   1 derivative-of-x4
   1 needs-conjugate
   1 sine-x-over-x