fix converter delegate

Author: tanner0101

Package.swift

@@ -3,6 +3,9 @@ import PackageDescription
 
 let package = Package(
     name: "fluent-postgres-driver",
+    platforms: [
+       .macOS(.v10_14)
+    ],
     products: [
         .library(name: "FluentPostgresDriver", targets: ["FluentPostgresDriver"]),
     ],

Sources/FluentPostgresDriver/PostgresConverterDelegate.swift

@@ -11,6 +11,8 @@ struct PostgresConverterDelegate: SQLConverterDelegate {
             return SQLRaw("BYTEA")
         case .datetime:
             return SQLRaw("TIMESTAMPTZ")
+        case .double:
+            return SQLRaw("DOUBLE PRECISION")
         default:
             return nil
         }