drop support of go 1.11 due to bad dep handling without mod support

.travis.yml

 matrix:
   include:
-  - language: go
-    go: 1.11.x
   - language: go
     go: 1.12.x
   - language: go

git-bug.go

@@ -8,9 +8,6 @@ package main
 
 import (
 	"github.com/MichaelMure/git-bug/commands"
-
-	// minimal go version is 1.11
-	_ "github.com/theckman/goconstraint/go1.11/gte"
 )
 
 func main() {

go.mod

 module github.com/MichaelMure/git-bug
 
-go 1.11
+go 1.12
 
 require (
 	github.com/99designs/gqlgen v0.10.3-0.20200209012558-b7a58a1c0e4b
@@ -25,7 +25,6 @@ require (
 	github.com/skratchdot/open-golang v0.0.0-20190402232053-79abb63cd66e
 	github.com/spf13/cobra v0.0.6
 	github.com/stretchr/testify v1.5.1
-	github.com/theckman/goconstraint v1.11.0
 	github.com/vektah/gqlparser v1.3.1
 	github.com/xanzy/go-gitlab v0.29.0
 	golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550