ci: update rust version for sgx

Author: Taowyoo

.travis.yml

@@ -21,14 +21,13 @@ rust:
    - stable
    - beta
    - nightly
-   - nightly-2019-07-01
-   - nightly-2019-10-28
+   - nightly-2021-03-25
 env:
   global:
     - RUST_BACKTRACE=1
     # 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-2019-07-01
-    - SGX_NIGHTLY=nightly-2019-10-28
+    - CORE_IO_NIGHTLY=nightly-2021-03-25
+    - SGX_NIGHTLY=nightly-2021-03-25
 script:
   - ./ct.sh