Rust Petstore server failure in Travis CI

Description

There's no change to the Rust petstore server samples but recently it started failing with the following errors:

   Compiling petstore_api v1.0.0 (file:///home/travis/build/OpenAPITools/openapi-generator/samples/server/petstore/rust-server)
error: lint unused_doc_comment has been renamed to unused_doc_comments
 --> examples/server_lib/mod.rs:6:5
  |
6 |     error_chain!{}
  |     ^^^^^^^^^^^^^^
  |
  = note: `-D renamed-and-removed-lints` implied by `-D warnings`
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: lint unused_doc_comment has been renamed to unused_doc_comments
 --> examples/server_lib/mod.rs:6:5
  |
6 |     error_chain!{}
  |     ^^^^^^^^^^^^^^
  |
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: lint unused_doc_comment has been renamed to unused_doc_comments
 --> examples/server_lib/mod.rs:6:5
  |
6 |     error_chain!{}
  |     ^^^^^^^^^^^^^^
  |
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: lint unused_doc_comment has been renamed to unused_doc_comments
 --> examples/server_lib/mod.rs:6:5
  |
6 |     error_chain!{}
  |     ^^^^^^^^^^^^^^
  |
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: aborting due to 4 previous errors

error: Could not compile `petstore_api`.
warning: build failed, waiting for other jobs to finish...

cc @frol @farcaller @bjgill

openapi-generator version

Latest master

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/379

Suggest a fix/enhancement

I've not looked into the details but maybe we should freeze the version used in the Travis CI tests for the time being.