Update dependencies, enable pruning for vendor/

So, `dep` got an nice new feature to remove tests and non-go files from
`vendor/`, and this brings the size of the vendor directory from ~300MiB
down to ~20MiB. We don that now.

vendor/github.com/mattn/go-isatty/example_test.go

@@ -1,18 +0,0 @@
-package isatty_test
-
-import (
-	"fmt"
-	"os"
-
-	"github.com/mattn/go-isatty"
-)
-
-func Example() {
-	if isatty.IsTerminal(os.Stdout.Fd()) {
-		fmt.Println("Is Terminal")
-	} else if isatty.IsCygwinTerminal(os.Stdout.Fd()) {
-		fmt.Println("Is Cygwin/MSYS2 Terminal")
-	} else {
-		fmt.Println("Is Not Terminal")
-	}
-}

vendor/github.com/mattn/go-isatty/isatty_others_test.go

@@ -1,19 +0,0 @@
-// +build !windows
-
-package isatty
-
-import (
-	"os"
-	"testing"
-)
-
-func TestTerminal(t *testing.T) {
-	// test for non-panic
-	IsTerminal(os.Stdout.Fd())
-}
-
-func TestCygwinPipeName(t *testing.T) {
-	if IsCygwinTerminal(os.Stdout.Fd()) {
-		t.Fatal("should be false always")
-	}
-}

vendor/github.com/mattn/go-isatty/isatty_windows_test.go

@@ -1,35 +0,0 @@
-// +build windows
-
-package isatty
-
-import (
-	"testing"
-)
-
-func TestCygwinPipeName(t *testing.T) {
-	tests := []struct {
-		name   string
-		result bool
-	}{
-		{``, false},
-		{`\msys-`, false},
-		{`\cygwin-----`, false},
-		{`\msys-x-PTY5-pty1-from-master`, false},
-		{`\cygwin-x-PTY5-from-master`, false},
-		{`\cygwin-x-pty2-from-toaster`, false},
-		{`\cygwin--pty2-from-master`, false},
-		{`\\cygwin-x-pty2-from-master`, false},
-		{`\cygwin-x-pty2-from-master-`, true}, // for the feature
-		{`\cygwin-e022582115c10879-pty4-from-master`, true},
-		{`\msys-e022582115c10879-pty4-to-master`, true},
-		{`\cygwin-e022582115c10879-pty4-to-master`, true},
-	}
-
-	for _, test := range tests {
-		want := test.result
-		got := isCygwinPipeName(test.name)
-		if want != got {
-			t.Fatalf("isatty(%q): got %v, want %v:", test.name, got, want)
-		}
-	}
-}