add a check for the minimal go version (1.8)

Gopkg.lock

@@ -320,6 +320,14 @@
   revision = "f35b8ab0b5a2cef36673838d662e249dd9c94686"
   version = "v1.2.2"
 
+[[projects]]
+  digest = "1:0ec412b28282c10ac6ce56039120741bb738814e4874d23c15fdff37f6aa7a62"
+  name = "github.com/theckman/goconstraint"
+  packages = ["go1.8/gte"]
+  pruneopts = "UT"
+  revision = "93babf24513d0e8277635da8169fcc5a46ae3f6a"
+  version = "v1.11.0"
+
 [[projects]]
   digest = "1:b24d38b282bacf9791408a080f606370efa3d364e4b5fd9ba0f7b87786d3b679"
   name = "github.com/urfave/cli"
@@ -471,6 +479,7 @@
     "github.com/spf13/cobra/doc",
     "github.com/stretchr/testify/assert",
     "github.com/stretchr/testify/require",
+    "github.com/theckman/goconstraint/go1.8/gte",
     "github.com/vektah/gqlgen/client",
     "github.com/vektah/gqlparser",
     "github.com/vektah/gqlparser/ast",

git-bug.go

@@ -5,7 +5,12 @@
 
 package main
 
-import "github.com/MichaelMure/git-bug/commands"
+import (
+	"github.com/MichaelMure/git-bug/commands"
+
+	// minimal go version is 1.8
+	_ "github.com/theckman/goconstraint/go1.8/gte"
+)
 
 func main() {
 	commands.Execute()

vendor/github.com/theckman/goconstraint/LICENSE

@@ -0,0 +1,9 @@
+This package is released under two software licenses (MIT | Public Domain):
+
+* The `scripts/` directory is released under the MIT License. This license can
+  be found in the `LICENSES` directory as mit.txt [1].
+* All other files are released to the Public Domain using The Unlicense . This
+  license can be found in the `LICENSES` directory as unlicense.txt [2].
+
+[1] https://github.com/theckman/goconstraint/blob/master/LICENSES/mit.txt
+[2] https://github.com/theckman/goconstraint/blob/master/LICENSES/unlicense.txt

vendor/github.com/theckman/goconstraint/LICENSES/unlicense.txt

@@ -0,0 +1,24 @@
+This is free and unencumbered software released into the public domain.
+
+Anyone is free to copy, modify, publish, use, compile, sell, or
+distribute this software, either in source code form or as a compiled
+binary, for any purpose, commercial or non-commercial, and by any
+means.
+
+In jurisdictions that recognize copyright laws, the author or authors
+of this software dedicate any and all copyright interest in the
+software to the public domain. We make this dedication for the benefit
+of the public at large and to the detriment of our heirs and
+successors. We intend this dedication to be an overt act of
+relinquishment in perpetuity of all present and future rights to this
+software under copyright law.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+OTHER DEALINGS IN THE SOFTWARE.
+
+For more information, please refer to <https://unlicense.org>

vendor/github.com/theckman/goconstraint/go1.8/gte/constraint.go

@@ -0,0 +1,8 @@
+// The contents of this file has been released in to the Public Domain.
+
+// Package gtego18 should only be used as a blank import. If imported, it
+// will only compile if the Go runtime version is >= 1.8.
+package gtego18
+
+// This will fail to compile if the Go runtime version isn't >= 1.8.
+var _ = __SOFTWARE_REQUIRES_GO_VERSION_1_8__

vendor/github.com/theckman/goconstraint/go1.8/gte/go18.go

@@ -0,0 +1,7 @@
+// The contents of this file has been released in to the Public Domain.
+
+// +build go1.8
+
+package gtego18
+
+const __SOFTWARE_REQUIRES_GO_VERSION_1_8__ = uint8(0)