Incorrect interp url in navbar on ajax nav

[jmcarp]

I'm not sure what the scope of this issue is, but here's a set of steps that reproduces the error:

• Open http://www.consumerfinance.gov/eregulations/1005-Subpart-A-Interp/2014-20681#1005-2-a-Interp-1
• Click "Appendix C" in the ToC
• Click "Subpart A" in the ToC again
• Hard-refresh the page
• 404!

For some reason, the url we're pushing to the browser history is

http://www.consumerfinance.gov/eregulations/1005-Subpart-A-In#1005-Subpart-A-Interp

but should be

http://www.consumerfinance.gov/eregulations/1005-Subpart-A-Interp/2014-20681#1005-2-a-Interp-1

FWIW, this seems to work correctly on https://github.com/eregs/regulations-site.