proxy: update Rust compiler to 1.27.0
# This reduces build time and produces binaries with debug symbols, at the expense of
# runtime perforamnce.
-ARG RUST_IMAGE=rust:1.26.2
+ARG RUST_IMAGE=rust:1.27.0
ARG RUNTIME_IMAGE=gcr.io/runconduit/base:2017-10-30.01
## Builds the proxy as incrementally as possible.
