replace libflate with flate2 (#476)

Cargo.toml

@@ -19,7 +19,7 @@ http = "0.1.15"
 hyper = "0.12.22"
 hyper-old-types = { version = "0.11", optional = true, features = ["compat"] }
 hyper-tls = { version = "0.3", optional = true }
-libflate = "0.1.18"
+flate2 = { version = "^1.0.7", default-features = false, features = ["rust_backend"] }
 log = "0.4"
 mime = "0.3.7"
 mime_guess = "2.0.0-alpha.6"
@@ -44,6 +44,7 @@ rustls = { version = "0.15", features = ["dangerous_configuration"], optional =
 env_logger = "0.6"
 serde_derive = "1.0"
 tokio-tcp = "0.1"
+libflate = "0.1"
 
 [features]
 default = ["default-tls"]