updated gemspec for v0.5.0
updated gemspec for v0.5.0

HaiTo/database_cleaner,awilkening/database_cleaner,0xCCD/database_cleaner,gentoo/database_cleaner,claptimes5/database_cleaner,yangc5/playlister-sinatra-v-000,yangc5/playlister-sinatra-v-000,yangc5/playlister-sinatra-v-000,DatabaseCleaner/database_cleaner,stanislaw/database_cleaner,custora/database_cleaner,DatabaseCleaner/database_cleaner,yangc5/playlister-sinatra-v-000,simonmorley/database_cleaner,avalonmediasystem/database_cleaner,ericraio/database_cleaner,glebtv/database_cleaner,fajrif/database_cleaner,yangc5/playlister-sinatra-v-000,eliotsykes/database_cleaner,yangc5/playlister-sinatra-v-000,reinaris/database_cleaner,amorphid/database_cleaner,yangc5/playlister-sinatra-v-000