travis: Remove Travis CI configuration

Travis hasn't worked in a while, is shutting down for most projects, and
we have already removed it from all public documentation.

Also all the functionality it provided is now provided by GitLab CI.

This patch removes the obsolete Travis configuration file.

.travis.yml

@@ -1,44 +0,0 @@
-# Configuration for https://travis-ci.org/
-
-language: go
-go_import_path: blitiri.com.ar/go/chasquid
-dist: bionic
-
-jobs:
-  include:
-    - name: "go 1.11 (debian stable)"
-      go: 1.11.x
-      # Enable Go modules explicitly on Go 1.11.
-      # Some packages update in backwards-incompatible ways assuming Go
-      # modules are in use, which can break the build unexpectedly.
-      env: GO111MODULE=on
-    - name: "go stable"
-      go: stable
-    - name: "go tip"
-      go: master
-  allow_failures:
-    # There is a bug in the Travis environment where it can't reach certain
-    # hosts that host Go package dependencies.
-    # This manifests only in Go 1.11, since subsequent versions use the public
-    # proxy which has no problems.
-    # Make the build optional until the issue is fixed.
-    - go: 1.11.x
-
-# This is needed because the repository has a Makefile, so travis won't invoke
-# "go get" by default.
-install:
-    - go get blitiri.com.ar/go/chasquid
-    - go get blitiri.com.ar/go/chasquid/cmd/chasquid-util
-    - go get blitiri.com.ar/go/chasquid/cmd/mda-lmtp
-    - go get blitiri.com.ar/go/chasquid/cmd/smtp-check
-    - go get blitiri.com.ar/go/chasquid/cmd/spf-check
-
-script:
-    - make all
-    - go test ./...
-    - go test -race ./...
-
-notifications:
-    email:
-        on_success: change
-        on_failure: always