use npm to download bats

.travis.yml

@@ -1,10 +1,7 @@
 language: bash
 os: linux
-service:
-  - docker
-before_script: docker pull bats/bats:latest
+before_script: npm install -g bats
 script:
   - ./shellcheck.sh
-  - export PATH="$PATH:${PWD}"
   - dotbare finit -u https://github.com/kazhala/dotfiles.git
-  - docker run -it -v "${PWD}:/code" bats/bats:latest --tap ./tests
+  - bats tests