Travis-CI : enable cache

[kangwonlee]

• To accelerate building

[kangwonlee]

• 8cbcf15 implemented caching
• 20 min → 10 min
• cache size : 4.8GB / branch (isn't it too big?)
• https://eng.localytics.com/best-practices-and-common-mistakes-with-travis-ci/ says 1GB is too large and may slow down the build; in this case, the cache is still saving time by half
• How to save time even further?

[kangwonlee]

#40