Update the repository url in Cargo.toml

Cargo.toml

@@ -13,7 +13,7 @@ authors = [
     "birktj"]
 license = "MIT"
 description = "TIFF decoding and encoding library in pure Rust"
-repository = "https://github.com/PistonDevelopers/image-tiff"
+repository = "https://github.com/image-rs/image-tiff"
 
 [dependencies]
 byteorder = "1.2"