Bump Go 1.15

Currently DC/OS CLI keeps backward compatibility for
macOS 10.10 Yosemite (2014) after updating Go version
we will drop it and support only macOS 10.12 Sierra or later.

Author: janisz

Diff for: .appveyor.yml

@@ -1,6 +1,6 @@
 install:
   - SET PATH=C:\msys64\mingw64\bin;c:\gopath\bin;%PATH%
-  - go get github.com/golangci/golangci-lint/cmd/golangci-lint@v1.27.0
+  - curl -sfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh| sh -s -- -b "c:\gopath\bin" v1.30.0
 
 build: off
 
@@ -10,9 +10,9 @@ environment:
   GOPATH: c:\gopath
   NO_DOCKER: 1
 
-stack: go 1.12
+stack: go 1.15
+image: Visual Studio 2019
 
 test_script:
-  - SET GO111MODULE=on
   - mingw32-make windows
   - mingw32-make test

Diff for: .travis.yml

@@ -4,15 +4,15 @@ os:
 
 language: go
 
-go: "1.12.x"
+go: "1.15.x"
 
 go_import_path: github.com/dcos/dcos-cli
 
 env:
  - NO_DOCKER=1 GO111MODULE=on
 
 before_install:
- - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $(go env GOPATH)/bin v1.21.0
+ - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $(go env GOPATH)/bin v1.30.0
 
 script:
  - make

Diff for: Dockerfile

@@ -1,8 +1,8 @@
-FROM golang:1.12
+FROM golang:1.15
 
 RUN go get -u \
     golang.org/x/lint/golint \
     github.com/awalterschulze/goderive \
     github.com/br-lewis/go-bindata/...
 
-RUN curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $(go env GOPATH)/bin v1.21.0
+RUN curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $(go env GOPATH)/bin v1.30.0