Add Travis ci

[thamaraiselvam]

I am working on it

[thamaraiselvam]

Reference: https://medium.com/flutter/test-flutter-apps-on-travis-3fd5142ecd8c

[luizfranca]

Hi @thamaraiselvam, in the PR #16 I added a .travis.yml file to configure travis on this project. Differently from the example in the reference, the file I added caches flutter on the machine, this should make the CI time much smaller (at least it did in my own project).