html_vignette() first exported in rmarkdown 0.2.65

I had rmarkdown 0.2.64 and got this on my first attempt to install `lazyeval` (for the purpose of updating `dplyr`):

```
Error: processing vignette 'benchmark.Rmd' failed with diagnostics:
'html_vignette' is not an exported object from 'namespace:rmarkdown'
Execution halted
```