Remove knitr from build instructions
Remove knitr from build instructions

JanSchulz/IRkernel,ibm-et/IRkernel,ChinaQuants/IRkernel,chendaniely/IRkernel,rgbkrk/IRkernel,elaOnMars/IRkernel,Phobia0ptik/IRkernel,mikecroucher/IRkernel,gdtm86/IRkernel,ibm-et/IRkernel