update to be compatable after 0.13

.terraform-version

@@ -1 +1 @@
-0.12.9
+0.13.5

private_subnets.tf

@@ -1,8 +1,8 @@
 resource "aws_subnet" "private" {
   count             = local.count_private
-  vpc_id            = "${aws_vpc.vpc.id}"
-  cidr_block        = "${cidrsubnet(var.vpc-cidr, length(var.vpc-azs) * 2, count.index + length(var.vpc-azs))}"
-  availability_zone = "${var.vpc-azs[count.index]}"
+  vpc_id            = aws_vpc.vpc.id
+  cidr_block        = cidrsubnet(var.vpc-cidr, length(var.vpc-azs) * 2, count.index + length(var.vpc-azs))
+  availability_zone = var.vpc-azs[count.index]
   tags              = merge({ Name = "${var.vpc-name}-private-${element(var.vpc-azs, count.index)}" }, var.common-tags)
 }
 
@@ -10,26 +10,26 @@ resource "aws_eip" "private-nat-eip" {
   count      = local.count_private
   vpc        = true
   tags       = merge({ Name = "${var.vpc-name}-nat-eip-${count.index}" }, var.common-tags)
-  depends_on = ["aws_internet_gateway.vpc"]
+  depends_on = [aws_internet_gateway.vpc]
 }
 
 resource "aws_nat_gateway" "private" {
   count         = local.count_private
-  allocation_id = "${element(aws_eip.private-nat-eip.*.id, count.index)}"
-  subnet_id     = "${element(aws_subnet.public.*.id, count.index)}"
-  depends_on    = ["aws_internet_gateway.vpc"]
+  allocation_id = element(aws_eip.private-nat-eip.*.id, count.index)
+  subnet_id     = element(aws_subnet.public.*.id, count.index)
+  depends_on    = [aws_internet_gateway.vpc]
   tags          = merge({ Name = "${var.vpc-name}-private-${element(var.vpc-azs, count.index)}" }, var.common-tags)
 }
 
 resource "aws_route" "nat_gateway" {
   count                  = local.count_private
-  route_table_id         = "${element(aws_route_table.private.*.id, count.index)}"
+  route_table_id         = element(aws_route_table.private.*.id, count.index)
   destination_cidr_block = "0.0.0.0/0"
-  nat_gateway_id         = "${element(aws_nat_gateway.private.*.id, count.index)}"
+  nat_gateway_id         = element(aws_nat_gateway.private.*.id, count.index)
 }
 
 resource "aws_route_table" "private" {
   count  = local.count_private
-  vpc_id = "${aws_vpc.vpc.id}"
+  vpc_id = aws_vpc.vpc.id
   tags   = merge({ Name = "${var.vpc-name}-private-${element(var.vpc-azs, count.index)}" }, var.common-tags)
 }