Change quick install instructions to be more explicit about checking out branch master.
Change quick install instructions to be more explicit about checking out branch
master.

walshjon/openmc,samuelshaner/openmc,shikhar413/openmc,johnnyliu27/openmc,liangjg/openmc,paulromano/openmc,liangjg/openmc,paulromano/openmc,wbinventor/openmc,paulromano/openmc,johnnyliu27/openmc,walshjon/openmc,amandalund/openmc,johnnyliu27/openmc,shikhar413/openmc,amandalund/openmc,mit-crpg/openmc,samuelshaner/openmc,lilulu/openmc,smharper/openmc,lilulu/openmc,amandalund/openmc,smharper/openmc,mjlong/openmc,kellyrowland/openmc,shikhar413/openmc,liangjg/openmc,shenqicang/openmc,shenqicang/openmc,smharper/openmc,nhorelik/openmc,walshjon/openmc,bhermanmit/openmc,johnnyliu27/openmc,walshjon/openmc,lilulu/openmc,sxds/opemmc,sxds/opemmc,paulromano/openmc,mit-crpg/openmc,kellyrowland/openmc,shikhar413/openmc,amandalund/openmc,wbinventor/openmc,smharper/openmc,samuelshaner/openmc,mit-crpg/openmc,wbinventor/openmc,mit-crpg/openmc,mjlong/openmc,wbinventor/openmc,bhermanmit/openmc,nhorelik/openmc,liangjg/openmc,samuelshaner/openmc