try fix CI

Author: Taowyoo

.travis.yml

@@ -11,10 +11,11 @@ os: linux
 dist: xenial
 addons:
   apt:
+    sources:
+    - sourceline: "deb http://apt.llvm.org/focal/ llvm-toolchain-focal-11 main"
+      key_url: "https://apt.llvm.org/llvm-snapshot.gpg.key"
     packages:
-    - llvm-3.8-dev
-    - libclang-3.8-dev
-    - clang-3.8
+    - clang-11
     - cmake
 rust:
    - stable
@@ -29,6 +30,5 @@ env:
     # re-pinned whenever core_io is updated to the latest nightly.
     - CORE_IO_NIGHTLY=nightly-2019-07-01
     - SGX_NIGHTLY=nightly-2019-10-28
-    - LLVM_CONFIG_PATH=llvm-config-3.8
 script:
   - ./ct.sh