Using --with-dep when Go Modules is enabled should show correct error

[gngchrs]

Description

Generating a new Buffalo project with Go Modules turned on errors when --with-dep is passed, as the two should not be used together. I noticed this when I had forgotten to unset GO111MODULE

Steps to Reproduce the Problem

1. Enable Go Modules with export GO111MODULE=on
2. Generate a new buffalo app with buffalo new coke --with-dep

Expected Behavior

Expected an error message telling me that Go Modules cannot be used with dep.

Actual Behavior

CLI exists with Error: exit status 1

Info

Please run buffalo info and paste the information below where it says "PASTE_HERE".

### Buffalo Version
v0.13.0

### App Information
Pwd=/Users/ganga/Dev/github/bplay/bufflog
Root=/Users/ganga/Dev/github/bplay/bufflog
GoPath=/Users/ganga/Dev/go
Name=bufflog
Bin=bin/bufflog
PackagePkg=bufflog
ActionsPkg=bufflog/actions
ModelsPkg=bufflog/models
GriftsPkg=bufflog/grifts
VCS=
WithPop=true
WithSQLite=false
WithDep=true
WithWebpack=false
WithYarn=false
WithDocker=true
WithGrifts=true
WithModules=true

### Go Version
go version go1.11.1 darwin/amd64

### Go Env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/ganga/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/ganga/Dev/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.11.1/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.11.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/ganga/Dev/github/bplay/bufflog/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/6q/6g96ggs515v9xj94kv5yk55h0000gn/T/go-build901320873=/tmp/go-build -gno-record-gcc-switches -fno-common"

### Node Version
v8.11.2

### NPM Version
6.4.1

### Yarn Version
1.10.1

### PostgreSQL Version
pg_ctl (PostgreSQL) 10.5

### MySQL Version
MySQL Not Found

### SQLite Version
3.19.3 2017-06-27 16:48:08 2b0954060fe10d6de6d479287dd88890f1bef6cc1beca11bc6cdb79f72e2377b

### Dep Version
dep:
 version     : v0.5.0
 build date  : 2018-07-26
 git hash    : 224a564
 go version  : go1.10.3
 go compiler : gc
 platform    : darwin/amd64
 features    : ImportDuringSolve=false

### Dep Status
/Users/ganga/Dev/github/bplay/bufflog is not within a known GOPATH/src
Usage:
  buffalo info [flags]

Flags:
  -h, --help   help for info

ERRO[0000] Error: exit status 1