Updated the npendulum example.

Fixes issue pydy#216.

- Removed the "bloggy" style text.
- Specified the dependencies.
- Included specific dependencies at the end of the notebook.
- Removed the ogv output, as Firefox now supports mp4 and matplotlib seems to
  fail on ogg: matplotlib/matplotlib#2764
- avconv is required as the writer.
- Changed to the linearizer introduced in SymPy 0.7.6.