Pin rust in s390x test job

Since the Rust 1.50.0 the s390x CI test job has been failing. This looks
to be caused by rust-lang/rust#80810. Until the issue is resolved in a
released version of rust this pins the rust version used in the job to
the previous release 1.49.0 which does not have this issue and should
work fine.

Author: mtreinish

.travis.yml

@@ -37,6 +37,8 @@ jobs:
       stage: Linux non-x86_64
       python: 3.7
       arch: s390x
+      before_script:
+        - rustup default 1.49.0
     - name: Python 3.7 Tests arm64 Linux
       stage: Linux non-x86_64
       python: 3.7