Update Rust versions under test from 1.30.1 to 1.37.0 (#5538)

Fixes build errors from using newer crate versions with an old Rust version.

tests/docker/languages/Dockerfile.testing.rust.big_endian.1_37_0

@@ -0,0 +1,15 @@
+FROM rust:1.37.0-slim-stretch as base
+RUN apt -qq update -y && apt -qq install -y \
+    gcc-mips-linux-gnu \
+    libexpat1 \
+    libmagic1 \
+    libmpdec2 \
+    libreadline7 \
+    qemu-user
+RUN rustup target add mips-unknown-linux-gnu
+WORKDIR /code
+ADD . .
+RUN cp flatc_debian_stretch flatc
+WORKDIR /code/tests
+RUN rustc --version
+RUN ./RustTest.sh mips-unknown-linux-gnu