env-logger: Fix hyper breaking Rust 1.26.0 build

Looks like hyper 0.12.26 (ironically) breaks the build on Rust 1.26.0.
Since we only depend on it for examples, I made our dependency version
more restrictive.

Signed-off-by: Eliza Weisman <[email protected]>

Author: hawkw

tokio-trace-env-logger/Cargo.toml

@@ -13,6 +13,6 @@ tokio-trace = { git = "https://github.com/tokio-rs/tokio" }
 tokio-trace-fmt = { path = "../tokio-trace-fmt" }
 tokio-trace-futures = { path = "../tokio-trace-futures" }
 tokio-trace-subscriber = { path = "../tokio-trace-subscriber" }
-hyper = "0.12"
+hyper = "=0.12.25"
 futures = "0.1"
 tokio = "0.1"