THRIFT-5158 Update Rust generator and Rust lib,test,tutorial to only support 2018 edition
Client: rs
Patch: Allen George

This closes #2078
diff --git a/build/docker/ubuntu-xenial/Dockerfile b/build/docker/ubuntu-xenial/Dockerfile
index 315b298..535db41 100644
--- a/build/docker/ubuntu-xenial/Dockerfile
+++ b/build/docker/ubuntu-xenial/Dockerfile
@@ -253,7 +253,7 @@
       ruby-bundler
 
 # Rust dependencies
-RUN curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain 1.34.0 -y
+RUN curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain 1.40.0 -y
 
 # Clean up
 RUN rm -rf /var/cache/apt/* && \