proxy: Update Rust to 1.26.2
Rust 1.26.2 addresses a correctness bug in the borrow checker.
See:
# This reduces build time and produces binaries with debug symbols, at the expense of
# runtime perforamnce.
-ARG RUST_IMAGE=rust:1.26.0
+ARG RUST_IMAGE=rust:1.26.2
ARG RUNTIME_IMAGE=gcr.io/runconduit/base:2017-10-30.01
## Builds the proxy as incrementally as possible.
