Enable bundler caching for travis

[Taher-Ghaleb]

Would be interested to know why bundler cache hasn't been enabled on Travis. Thank you.

[coveralls]

Coverage remained the same at 100.0% when pulling 62dea09 on Taher-Ghaleb:master into 90e0914 on tom-lord:master.

[tom-lord]

This project was created in 2014. Cacheing was added to Travis in December 2013, but only for private repositories; it wasn't until 2016 that this feature became available for public, non-containerised repos like this.

Also, given the speed of running tests, (I'd normally just run them all locally instead of relying on CI!), and how often the project gets updated these days, it's hardly been a priority for me to change the config 😉

But thanks for pointing this out; I'll merge the improvement 🥳