Introduce Travis CI (#46)
Introduce Travis CI (#46)

* Introduce travis CI

so that we can avoid unexpected spec failure before merging.
e.g. https://github.com/nomad/venice/pull/44

* Remove before_install and script section

Because 

- it installs the latest bundler automatically
- `rake` runs RSpec by default.

ref: https://github.com/nomad/venice/pull/46#issuecomment-348253146

* Ruby 2.4.3 has been released

https://www.ruby-lang.org/en/news/2017/12/14/ruby-2-4-3-released/

nomad/venice