$ gem install rdiscount -v 1.6
ERROR:  Could not find a valid gem 'rdiscount' (= 1.6) in any repository
ERROR:  Possible alternatives: rdiscount


$ gem list rdiscount

*** LOCAL GEMS ***

rdiscount (2.1.7.1)




$ gem list -r rdiscount

*** REMOTE GEMS ***

chaptastic-rdiscount (1.4.1)
forem-rdiscount (1.0.0)
patcito-rdiscount (1.6.9)
pixeltrix-rdiscount (1.3.4)
rack-rdiscount (0.0.3)
rdiscount (2.1.7.1)
rdiscount-dsc (1.6.9)
rdiscount_wm (3.1.0.0)
rtomayko-rdiscount (1.3.5)
sinatra-rdiscount (1.0.0.6)
sinatra-simple-rdiscount (0.0.1)
wbzyl-sinatra-rdiscount (0.10.1)



gem install rdiscount -v 1.6.9