Provide correct install command for prerelese gems.
Provide correct install command for prerelese gems.

Resolves #19

geminabox/geminabox,hooroo/geminabox,geminabox/geminabox,legitscript/geminabox,tenex/rails-assets,legitscript/geminabox,appfolio/geminabox,VodafoneAustralia/geminabox,legitscript/geminabox,geminabox/geminabox,sewichi/geminabox,sujithrs/geminabox,bibendi/geminabox,asiazhang/geminabox,sujithrs/geminabox,sewichi/geminabox,asiazhang/geminabox,sujithrs/geminabox,brennovich/geminabox,nicolasleger/geminabox,VodafoneAustralia/geminabox,hooroo/geminabox,legitscript/geminabox,tenex/rails-assets,hooroo/geminabox,abak-press/geminabox,hooroo/geminabox,sewichi/geminabox,tenex/rails-assets,nicolasleger/geminabox,sujithrs/geminabox,abak-press/geminabox,abak-press/geminabox,appfolio/geminabox,brennovich/geminabox,nicolasleger/geminabox,appfolio/geminabox,VodafoneAustralia/geminabox,geminabox/geminabox,asiazhang/geminabox,sewichi/geminabox,nicolasleger/geminabox,bibendi/geminabox,brennovich/geminabox,bibendi/geminabox,asiazhang/geminabox,VodafoneAustralia/geminabox,abak-press/geminabox,bibendi/geminabox,appfolio/geminabox