Update MAKEFILE to check knitr version

[sritchie73]

make preview will only compile the code blocks in the challenges and callouts correctly if knitr is version 1.10.12 or higher. Currently, this requires installing from github: (devtools::install_github("yihui/knitr")).

I've written an R script that will throw an error if the knitr version is too low (tools/check_knitr_version.R) in Pull Request #41 , but I wasn't able to successfully incorporate it into the Makefile to prevent future contributors from clobbering the code block rendering.

[sritchie73]

Thoughts on this approach @jdblischak ? Does r-novice-inflammation have code blocks in any of its challenges?

[aammd]

Moving this discussion over to the lesson-template repo. Hopefully we will be able to change the One True Makefile to support this behaviour, and then I will include those changes here.

[jdblischak]

I think this issue can be closed now that the following PR has been incorporated into the lesson-template:

swcarpentry/DEPRECATED-lesson-template#271

[naupaka]

Yup. Good call.