Fix test_pbkdf2. Fixing #295.

CryptoSwiftTests/PBKDF.swift

@@ -29,7 +29,7 @@ class PBKDF: XCTestCase {
     func test_pbkdf2() {
         let password: Array<UInt8> = "s33krit".utf8.map {$0}
         let salt: Array<UInt8> = "nacl".utf8.map {$0}
-        let value = try! PKCS5.PBKDF2(password: password, salt: salt, iterations: 2, keyLength: 123, variant: .sha1).calculate()
+        let value = try! PKCS5.PBKDF2(password: password, salt: salt, iterations: 2, keyLength: 140, variant: .sha1).calculate()
         XCTAssert(value.toHexString() == "a53cf3df485e5cd91c17c4978048e3ca86e03cced5f748fb55eff9c1edfce7f9f490c0beee768b85c1ba14ec5750cf059fea52565ffd9e4f9dba01c5c953955e7f1012b6a9eb40629ce767982e598df9081048e22781b35187c16d61ac43f69b88630a9e80233b4c58bdc74ea5c06b5bb1b2c2a86e3ddc2775b852c4508ac85a6a47c0e23a3d8dc6e4dca583", "PBKDF2 fail")
     }