travis: Remove references in documentation, allow to fail

The Travis tests don't work due to a Travis networking issue reaching
some external hosts.

Until it is fixed, remove references from the documentation, since its
output is misleading.

We also allow it to fail, which makes the GitHub UI not show commits as
having failed tests because of Travis.

Since the previous commit, GitLab CI does almost all the same tests, so
there should be no significant loss of coverage anyway.

.travis.yml

@@ -2,22 +2,27 @@
 
 language: go
 go_import_path: blitiri.com.ar/go/chasquid
-dist: trusty
-sudo: false
+dist: bionic
 
-go:
-    # Check against the version in Debian stable.
-    - 1.11.x
-    - stable
-    - master
-
-env:
-  # 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.
-  # We can remove this once we no longer support Go 1.11, as it is the default
-  # in the newer versions.
-  - GO111MODULE=on
+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.
@@ -37,9 +42,3 @@ notifications:
     email:
         on_success: change
         on_failure: always
-    irc:
-        channels:
-          - "ircs://chat.freenode.net:7070/#chasquid"
-        use_notice: true
-        on_success: change
-        on_failure: change