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` (on the way to installing `dplyr`):

```
Error: processing vignette 'benchmark.Rmd' failed with diagnostics:
'html_vignette' is not an exported object from 'namespace:rmarkdown'
Execution halted
```
Update DESCRIPTION
Merge pull request #13 from jennybc/patch-1

html_vignette() first exported in rmarkdown 0.2.65