vendor: update all dependencies

* Update all dependencies
  * Remove all `[[constraint]]` from Gopkg.toml
  * Add in the minimum number of `[[override]]` to build
  * Remove go get of github.com/inconshreveable/mousetrap as it is vendored
  * Update docs with new policy on constraints

vendor/cloud.google.com/go/bigtable/bttest/inmem.go

@@ -121,7 +121,7 @@ func (s *server) CreateTable(ctx context.Context, req *btapb.CreateTableRequest)
 	s.mu.Lock()
 	if _, ok := s.tables[tbl]; ok {
 		s.mu.Unlock()
-		return nil, grpc.Errorf(codes.AlreadyExists, "table %q already exists", tbl)
+		return nil, status.Errorf(codes.AlreadyExists, "table %q already exists", tbl)
 	}
 	s.tables[tbl] = newTable(req)
 	s.mu.Unlock()
@@ -151,7 +151,7 @@ func (s *server) GetTable(ctx context.Context, req *btapb.GetTableRequest) (*bta
 	tblIns, ok := s.tables[tbl]
 	s.mu.Unlock()
 	if !ok {
-		return nil, grpc.Errorf(codes.NotFound, "table %q not found", tbl)
+		return nil, status.Errorf(codes.NotFound, "table %q not found", tbl)
 	}
 
 	return &btapb.Table{
@@ -177,7 +177,7 @@ func (s *server) ModifyColumnFamilies(ctx context.Context, req *btapb.ModifyColu
 	tbl, ok := s.tables[req.Name]
 	s.mu.Unlock()
 	if !ok {
-		return nil, grpc.Errorf(codes.NotFound, "table %q not found", req.Name)
+		return nil, status.Errorf(codes.NotFound, "table %q not found", req.Name)
 	}
 
 	tbl.mu.Lock()
@@ -186,7 +186,7 @@ func (s *server) ModifyColumnFamilies(ctx context.Context, req *btapb.ModifyColu
 	for _, mod := range req.Modifications {
 		if create := mod.GetCreate(); create != nil {
 			if _, ok := tbl.families[mod.Id]; ok {
-				return nil, grpc.Errorf(codes.AlreadyExists, "family %q already exists", mod.Id)
+				return nil, status.Errorf(codes.AlreadyExists, "family %q already exists", mod.Id)
 			}
 			newcf := &columnFamily{
 				name:   req.Name + "/columnFamilies/" + mod.Id,