travis-ci is ceased

[daun-io]

https://travis-ci.org/github/konlpy/konlpy

Two years ago, we were happy to use travis-ci which helped us to test our code on various python distributions and environments.
However it seems they are no longer free even for the free and open source projects.
Therefore we have to slowly migrate our ci environment to Github Actions in order to review PRs with confidence and write better code.