Fix the build for latest knitr

[jeroen]

Fixes the build to work with the latest version of knitr:

── R CMD build ─────────────────────────────────────────────────────────────────
* checking for file ‘.../DESCRIPTION’ ... OK
* preparing ‘shiny.semantic’:
* checking DESCRIPTION meta-information ... OK
* installing the package to build vignettes
* creating vignettes ... ERROR
Error: --- re-building ‘fomantic_js.Rmd’ using rmarkdown
Error: Error: processing vignette 'fomantic_js.Rmd' failed with diagnostics:
The 'rmarkdown' package should be installed and declared as a dependency of the 'shiny.semantic' package (e.g., in the 'Suggests' field of DESCRIPTION), because the latter contains vignette(s) built with the 'rmarkdown' package. Please see https://github.com/yihui/knitr/issues/1864 for more information.
--- failed re-building ‘fomantic_js.Rmd’

--- re-building ‘basics.Rmd’ using knitr
Error: Error: processing vignette 'basics.Rmd' failed with diagnostics:
The 'markdown' package should be installed and declared as a dependency of the 'shiny.semantic' package (e.g., in the 'Suggests' field of DESCRIPTION), because the latter contains vignette(s) built with the 'markdown' package. Please see https://github.com/yihui/knitr/issues/1864 for more information.
--- failed re-building ‘basics.Rmd’

--- re-building ‘intro.Rmd’ using knitr
Error: Error: processing vignette 'intro.Rmd' failed with diagnostics:
The 'markdown' package should be installed and declared as a dependency of the 'shiny.semantic' package (e.g., in the 'Suggests' field of DESCRIPTION), because the latter contains vignette(s) built with the 'markdown' package. Please see https://github.com/yihui/knitr/issues/1864 for more information.
--- failed re-building ‘intro.Rmd’

--- re-building ‘semantic_integration.Rmd’ using knitr
Error: Error: processing vignette 'semantic_integration.Rmd' failed with diagnostics:
The 'markdown' package should be installed and declared as a dependency of the 'shiny.semantic' package (e.g., in the 'Suggests' field of DESCRIPTION), because the latter contains vignette(s) built with the 'markdown' package. Please see https://github.com/yihui/knitr/issues/1864 for more information.
--- failed re-building ‘semantic_integration.Rmd’

SUMMARY: processing the following files failed:
  ‘fomantic_js.Rmd’ ‘basics.Rmd’ ‘intro.Rmd’ ‘semantic_integration.Rmd’

[dokato]

Thanks @jeroen !