Changed test timeout to 30 seconds because it keeps failing on travis despite not failing locally

Tests/KingfisherTests/ImageCacheTests.swift

@@ -238,7 +238,7 @@ class ImageCacheTests: XCTestCase {
             expectation.fulfill()
         }
         
-        self.waitForExpectationsWithTimeout(20, handler: nil)
+        self.waitForExpectationsWithTimeout(30, handler: nil)
     }
     
     func testCleanDiskCacheNotification() {