%i記法だとなぜかHerokuのbundle installでエラーが出るので素直に[]で書く

https://travis-ci.org/CodeforOkinawa/cal5374/builds/44598848#L193

creating application archive
uploading application archive
triggering new deployment
-----> Ruby app detected
-----> Compiling Ruby/Rails
 !
 !     There was an error parsing your Gemfile, we cannot continue
 !     Gemfile syntax error on line 6: unknown type of %string
 !     gem 'sqlite3', group: %i(development test)
 !     ^
 !     on line 6: syntax error, unexpected $end
 !     gem 'sqlite3', group: %i(development test)
 !     ^