Drop Go 1.11 and 1.12

Between Go 1.12 and Go 1.13, go.sum's checksum algorithm has been
changed. We cannot have single go.sum for the two versions.

According to https://golang.org/doc/devel/release.html; Go 1.11 and
Go.1.12 are no longer supported, after Go 1.14;

> Each major Go release is supported until there are two newer major
> releases.

Given the fact we only run integration tests with Go 1.13 on BuildKite,
it would be okay to remove the older Go versions from Travis and
GitHub Actions.

Signed-off-by: Kazuyoshi Kato <[email protected]>

Author: kzys

.github/workflows/build.yaml

@@ -17,7 +17,7 @@ jobs:
 
     strategy:
       matrix:
-        go: ['1.11', '1.12', '1.13', '1.14']
+        go: ['1.13', '1.14']
       # Build all variants regardless of failures
       fail-fast: false
     name: Go ${{ matrix.go }}

.travis.yml

@@ -2,9 +2,8 @@ dist: xenial
 language: go
 
 go:
+  - 1.14.x
   - 1.13.x
-  - 1.12.x
-  - 1.11.x
 
 env:
   - GO111MODULE=on