Update OS in CI

.travis.yml

@@ -8,17 +8,11 @@ branches:
     - master
 language: rust
 os: linux
-dist: xenial
+dist: focal
 addons:
   apt:
-    sources:
-    - sourceline: "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-11 main"
-      key_url: "https://apt.llvm.org/llvm-snapshot.gpg.key"
     packages:
-    - llvm-3.9-dev
-    - libclang-3.9-dev
-    - clang-3.9
-    - clang-11
+    - clang
     - cmake
     - qemu-user
 rust:
@@ -34,7 +28,6 @@ env:
     # Pinned to this particular nightly version because of core_io. This can be
     # re-pinned whenever core_io is updated to the latest nightly.
     - CORE_IO_NIGHTLY=nightly-2021-03-25
-    - LLVM_CONFIG_PATH=llvm-config-3.9
 jobs:
   include:
     # Test additional Rust toolchains on x86_64