ci(travis): Bump golangci-lint version

The current recommended installation steps use githubusercontent.com
instead of goreleaser.com. There has been a reported issue in which the
installer code differed between the two sites.
golangci/golangci-lint#575

Additionally, pin to a specific tagged version of the install script to
ensure reproducible installations of the tool.

Use version 1.19.1.

The latest two releases (1.21.0, 1.20.0) have a flaky bug (at least on
macOS) that prevents the linter to run while still exiting with code 0
(success).

```
~/s/sentry-go ❯❯❯ golangci-lint run
WARN [runner] Can't run linter goanalysis_metalinter: assign: failed
prerequisites: [email protected]/getsentry/sentry-go/echo
~/s/sentry-go ❯❯❯ golangci-lint run
WARN [runner] Can't run linter goanalysis_metalinter: interfacer: failed
prerequisites: [email protected]/getsentry/sentry-go/echo
```

See
- golangci/golangci-lint#866
- golangci/golangci-lint#827

.travis.yml

@@ -10,7 +10,7 @@ env:
   - GO111MODULE=on GOFLAGS=-mod=readonly
 
 before_install:
-  - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $(go env GOPATH)/bin v1.15.0
+  - curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/v1.19.1/install.sh | sh -s -- -b $(go env GOPATH)/bin v1.19.1
 
 script:
   - golangci-lint run