Update required xfun version

[MichaelChirico]

As noted here:

35df6f8

The current {knitr} test suite assumes {xfun} >= 0.49

xfun::html_escape('"40"')
# [1] ""40""

Of course this is a behavior only in tests, so we don't technically need to bump the version; at a minimum we could put a branch in the test based on {xfun} version. But this approach seems fine + much simpler.

[yihui]

Thank you!