Updated Booktype version in INSTALL file to 1.6
Updated Booktype version in INSTALL file to 1.6

kronoscode/Booktype,eos87/Booktype,ride90/Booktype,kronoscode/Booktype,danielhjames/Booktype,olegpshenichniy/Booktype,olegpshenichniy/Booktype,MiczFlor/Booktype,danielhjames/Booktype,rob-hills/Booktype,okffi/booktype,kronoscode/Booktype,okffi/booktype,danielhjames/Booktype,danielhjames/Booktype,kronoscode/Booktype,eos87/Booktype,eos87/Booktype,eos87/Booktype,sourcefabric/Booktype,rob-hills/Booktype,MiczFlor/Booktype,sourcefabric/Booktype,rob-hills/Booktype,okffi/booktype,ride90/Booktype,olegpshenichniy/Booktype,btat/Booktype,olegpshenichniy/Booktype,MiczFlor/Booktype,sourcefabric/Booktype,MiczFlor/Booktype,ride90/Booktype,sourcefabric/Booktype,danielhjames/Booktype,okffi/booktype,sourcefabric/Booktype,ride90/Booktype,MiczFlor/Booktype,ride90/Booktype,btat/Booktype,btat/Booktype,olegpshenichniy/Booktype,rob-hills/Booktype,btat/Booktype,eos87/Booktype,kronoscode/Booktype,okffi/booktype