Add github source for golang/x to Gopkg.toml

Prevents removal of source in lock file.

Author: danielnelson

Gopkg.lock

@@ -189,10 +189,12 @@
 [[constraint]]
   name = "golang.org/x/net"
   branch = "master"
+  source = "https://github.com/golang/net.git"
 
 [[constraint]]
   name = "golang.org/x/sys"
   branch = "master"
+  source = "https://github.com/golang/sys.git"
 
 [[constraint]]
   name = "google.golang.org/grpc"
@@ -237,6 +239,7 @@
 [[constraint]]
   branch = "master"
   name = "golang.org/x/oauth2"
+  source = "https://github.com/golang/oauth2.git"
 
 [[constraint]]
   branch = "master"
@@ -265,3 +268,15 @@
 [[constraint]]
   name = "github.com/go-logfmt/logfmt"
   version = "0.4.0"
+
+[[override]]
+  name = "golang.org/x/crypto"
+  source = "https://github.com/golang/crypto.git"
+
+[[override]]
+  name = "golang.org/x/sync"
+  source = "https://github.com/golang/sync.git"
+
+[[override]]
+  name = "golang.org/x/text"
+  source = "https://github.com/golang/text.git"