Add missing public initializer to CurlyProvider

...and change the test to use regular import instead of `@testable` so this kind of stuff never happens again.

Sources/CurlyClient/CurlyProvider.swift

@@ -1,6 +1,9 @@
 import Vapor
 
 public class CurlyProvider: Provider {
+    public init() {
+    }
+
     public func register(_ services: inout Services) throws {
         services.register(CurlyClient.self)
         services.register { _ in CurlyOptionStorage() }

Tests/CurlyClientTests/CurlyClientTests.swift

@@ -1,7 +1,7 @@
 import XCTest
 import NIO
 import Vapor
-@testable import CurlyClient
+import CurlyClient
 
 final class CurlyClientTests: XCTestCase {
     private func testApplication() throws -> Application {