Have make check for recent knitr versions on R lessons

[aammd]

As pointed out by @sritchie73 in swcarpentry/r-novice-gapminder#42, presently the MAKEFILE doesn't check for recent versions of knitr. He has a solution to this problem (from swcarpentry/r-novice-gapminder#41). However, our makefile hasn't been modified to run this script. I understand that changes to the MAKEFILE ought to happen in this repository. What do you guys think of this approach? Could we make these changes, then pull them into r-novice-gapminder?

[fmichonneau]

I created a PR for this: #271

I put the check in the Makefile with the rule to build the Rmd files such that lessons that are not R-based don't have to worry about it. If there is a better approach, don't hesitate to let me know.