chore(travis): speedup travis build
chore(travis): speedup travis build

cache the gems
http://qiita.com/oh_rusty_nail/items/374c30ecc5aa9997d1f3#3-1

tako-black/jser.github.io,nhiroki/jser.github.io,jser/jser.github.io,jser/jser.github.io,nhiroki/jser.github.io,nhiroki/jser.github.io,tako-black/jser.github.io,kohei-takata/jser.github.io,jser/jser.github.io,kohei-takata/jser.github.io,tako-black/jser.github.io,kohei-takata/jser.github.io,nhiroki/jser.github.io,UYEONG/jser.github.io,techhtml/jser.github.io,techhtml/jser.github.io,tako-black/jser.github.io,UYEONG/jser.github.io,UYEONG/jser.github.io,jser/jser.github.io,kohei-takata/jser.github.io,techhtml/jser.github.io,techhtml/jser.github.io