Switch to golang 1.13, drop unsupported versions

Run CI with go 1.13 and 1.14 (aka "stable").

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.travis.yml

@@ -1,25 +1,24 @@
 dist: bionic
 language: go
 go:
-  - 1.12.x
+  - 1.14.x
   - 1.13.x
-  - 1.x
   - tip
 
 matrix:
   include:
-    - go: 1.x
+    - go: 1.13.x
       name: "verify-dependencies"
       script:
         - make verify-dependencies
-    - go: 1.12.x
+    - go: 1.13.x
       name: "cgroup-systemd"
       env:
         - RUNC_USE_SYSTEMD=1
       script:
         - make BUILDTAGS="${BUILDTAGS}" all
         - sudo PATH="$PATH" make localintegration RUNC_USE_SYSTEMD=1
-    - go: 1.12.x
+    - go: 1.13.x
       name: "cgroup-v2"
       env:
         - VIRTUALBOX_VERSION=6.0

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.12-stretch
+FROM golang:1.13-stretch
 
 RUN dpkg --add-architecture armel \
     && dpkg --add-architecture armhf \