Drop Go 1.12, require at least 1.13

.travis.yml

@@ -3,14 +3,6 @@ sudo: false
 
 matrix:
   include:
-    - os: linux
-      go: "1.12.x"
-      env: RESTIC_TEST_FUSE=0 RESTIC_TEST_CLOUD_BACKENDS=0
-      cache:
-        directories:
-          - $HOME/.cache/go-build
-          - $HOME/gopath/pkg/mod
-
     - os: linux
       go: "1.13.x"
       env: RESTIC_TEST_FUSE=0 RESTIC_TEST_CLOUD_BACKENDS=0

doc/020_installation.rst

@@ -245,7 +245,7 @@ From Source
 ***********
 
 restic is written in the Go programming language and you need at least
-Go version 1.12. Building restic may also work with older versions of Go,
+Go version 1.13. Building restic may also work with older versions of Go,
 but that's not supported. See the `Getting
 started <https://golang.org/doc/install>`__ guide of the Go project for
 instructions how to install Go.