travis: remove testing with go 1.x

Travis and CircleCI should use the same Go version(s).

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

.circleci/config.yml

@@ -3,6 +3,8 @@ version: 2
 
 jobs:
   test:
+    # Whenever the Go version is updated here, .travis.yml should also be
+    # updated.
     docker:
     - image: circleci/golang:1.10
     working_directory: /go/src/github.com/prometheus/prometheus

.travis.yml

@@ -2,9 +2,10 @@ sudo: false
 
 language: go
 
+# Whenever the Go version is updated here, .circleci/config.yml should also be
+# updated.
 go:
 - 1.10.x
-- 1.x
 
 go_import_path: github.com/prometheus/prometheus