chore(dependencies): update to futures alpha 18

Closes #1897

Cargo.toml

@@ -23,9 +23,9 @@ include = [
 
 [dependencies]
 bytes = "0.4.4"
-futures-core-preview = { version = "0.3.0-alpha.17" }
-futures-channel-preview = { version = "0.3.0-alpha.17" }
-futures-util-preview = { version = "0.3.0-alpha.17" }
+futures-core-preview = { version = "0.3.0-alpha.18" }
+futures-channel-preview = { version = "0.3.0-alpha.18" }
+futures-util-preview = { version = "0.3.0-alpha.18" }
 http = "0.1.15"
 http-body = "0.1"
 httparse = "1.0"
@@ -37,7 +37,7 @@ net2 = { version = "0.2.32", optional = true }
 pin-utils = "0.1.0-alpha.4"
 time = "0.1"
 tokio = { version = "0.2.0-alpha.2", optional = true, default-features = false, features = ["rt-full"] }
-tokio-buf = "0.2.0-alpha.1"
+#tokio-buf = "0.2.0-alpha.1"
 tokio-executor = "0.2.0-alpha.2"
 tokio-io = "0.2.0-alpha.2"
 tokio-sync = "0.2.0-alpha.2"