Convert `=&gt;` to the more semantic `&rarr;`.

`&rarr;` is defined in the HTML 4 spec [1]:

  <!ENTITY rarr CDATA "&#8594;" -- rightwards arrow, U+2192 ISOnum -->

This should probably wait until after an lxml migration (see
discussion in #168) so we don't confuse Python's stdlib xml parser.

[1]: http://www.w3.org/TR/html4/sgml/entities.html