clean(travis): remove shellcheck from before_install (#947)

.travis.yml

@@ -18,13 +18,10 @@ matrix:
       go: 1.12.x
       script: diff -u <(echo -n) <(gofmt -d -s .)
 
-before_install:
-  - mkdir -p bin
-  - curl -Lso bin/shellcheck https://github.com/caarlos0/shellcheck-docker/releases/download/v0.6.0/shellcheck
-  - chmod +x bin/shellcheck
-  - go get -u github.com/kyoh86/richgo
+before_install: go get -u github.com/kyoh86/richgo
+
 script:
-  - PATH=$PATH:$PWD/bin richgo test -v ./...
+  - richgo test -v ./...
   - go build
   - if [ -z $NOVET ]; then
       diff -u <(echo -n) <(go vet . 2>&1 | grep -vE 'ExampleCommand|bash_completions.*Fprint');