Add "bundle exec rbs collection init" before install command

[unasuke]

"rbs collection install" requires rbs_collection.yaml file, but current processes doesn't generate its.

(If you don't mind, could you label it "hacktoberfest-accepted"?)

[pocke]

Thanks! And sorry for the delay because I completely forgot this PR...

[unasuke]

😍 😍 😍 Thanks!