%s/code/erb/g

This changes the ERB template parser to use `<erb>` rather than `<code>`
to designate code blocks in a template.

For example:

    <%= render template: 'foo/bar' %>

Used to parse as:

    <code erb-loud> render template: 'foo/bar' </code>

But not parses as:

    <erb erb-loud> render template: 'foo/bar' </erb>

This fixes a number of problems that result from using `<code>`:

* There may be </code> tags in the original ERB, which would be turned
  into `%>`
  (Fixes #101)

* Any <code> tags in the <head> of a document would be moved to the
  body, since <code> is a real HTML tag, and isn't allowed in the head.
  This happens in nokogiri under jRuby or libXML 2.9.0
  (Fixes #84, Fixes #100)

Unfortunately this breaks any existing deface selectors looking for
code, which are somewhat common. Selectors should be updated to cover
both code and erb:

    selector: "code:contains('my_method'), erb:contains('my_method')"
test for head code tags