fix bug in mathjax

previously, if from IPython.external.mathjax import * was called, it
would cause an error, since the dest variable was renamed to
default_dest

IPython/external/mathjax.py

@@ -259,7 +259,7 b' def main() :'
 if __name__ == '__main__' :
     sys.exit(main())
 
-__all__ = ['install_mathjax', 'main', 'dest']
+__all__ = ['install_mathjax', 'main', 'default_dest']
 
 """
 Test notes: