proxy: Update to Rust 1.26.0
Notably, this adds support for `impl Trait`.
More info at
# This reduces build time and produces binaries with debug symbols, at the expense of
# runtime perforamnce.
-ARG RUST_IMAGE=rust:1.25.0
+ARG RUST_IMAGE=rust:1.26.0
ARG RUNTIME_IMAGE=gcr.io/runconduit/base:2017-10-30.01
## Builds the proxy as incrementally as possible.
