Update dependencies (#5518)

vendor/gopkg.in/src-d/go-billy.v4/.gitignore

@@ -2,4 +2,3 @@
 /vendor
 Gopkg.lock
 Gopkg.toml
-go.sum

vendor/gopkg.in/src-d/go-billy.v4/.travis.yml

@@ -11,6 +11,7 @@ install:
 
 script:
   - make test-coverage
+  - ./.ci/test-building-binaries-for-supported-os.sh
 
 after_success:
   - bash <(curl -s https://codecov.io/bash)

vendor/gopkg.in/src-d/go-billy.v4/README.md

@@ -1,4 +1,4 @@
-# go-billy [![GoDoc](https://godoc.org/gopkg.in/src-d/go-billy.v4?status.svg)](https://godoc.org/gopkg.in/src-d/go-billy.v4) [![Build Status](https://travis-ci.org/src-d/go-billy.svg)](https://travis-ci.org/src-d/go-billy) [![Build status](https://ci.appveyor.com/api/projects/status/vx2qn6vlakbi724t?svg=true)](https://ci.appveyor.com/project/mcuadros/go-billy) [![codecov](https://codecov.io/gh/src-d/go-billy/branch/master/graph/badge.svg)](https://codecov.io/gh/src-d/go-billy)
+# go-billy [![GoDoc](https://godoc.org/gopkg.in/src-d/go-billy.v4?status.svg)](https://godoc.org/gopkg.in/src-d/go-billy.v4) [![Build Status](https://travis-ci.com/src-d/go-billy.svg)](https://travis-ci.com/src-d/go-billy) [![Build status](https://ci.appveyor.com/api/projects/status/vx2qn6vlakbi724t?svg=true)](https://ci.appveyor.com/project/mcuadros/go-billy) [![codecov](https://codecov.io/gh/src-d/go-billy/branch/master/graph/badge.svg)](https://codecov.io/gh/src-d/go-billy)
 
 The missing interface filesystem abstraction for Go.
 Billy implements an interface based on the `os` standard library, allowing to develop applications without dependency on the underlying storage. Makes it virtually free to implement mocks and testing over filesystem operations.

vendor/gopkg.in/src-d/go-billy.v4/osfs/os_posix.go

@@ -3,19 +3,19 @@
 package osfs
 
 import (
-	"syscall"
+	"golang.org/x/sys/unix"
 )
 
 func (f *file) Lock() error {
 	f.m.Lock()
 	defer f.m.Unlock()
 
-	return syscall.Flock(int(f.File.Fd()), syscall.LOCK_EX)
+	return unix.Flock(int(f.File.Fd()), unix.LOCK_EX)
 }
 
 func (f *file) Unlock() error {
 	f.m.Lock()
 	defer f.m.Unlock()
 
-	return syscall.Flock(int(f.File.Fd()), syscall.LOCK_UN)
+	return unix.Flock(int(f.File.Fd()), unix.LOCK_UN)
 }