remove travis ci

.travis.yml

@@ -1,22 +0,0 @@
-sudo: false
-dist: trusty
-language: rust
-cache: cargo
-rust:
-  - 1.44.0
-branches:
-  only:
-  - master
-before_script:
-  - (cargo install mdbook --vers 0.4.1 --force || true)
-script:
-  - mdbook build
-  - echo 'nojekyll' > book/.nojekyll
-deploy:
-  provider: pages
-  local-dir: book
-  skip-cleanup: true
-  github-token: $GITHUB_TOKEN
-  keep-history: false
-  on:
-    branch: master