Workaround for broken initialization script of travis

Author: taiki-e

.travis.yml

@@ -28,10 +28,16 @@ matrix:
     - name: cargo test
       rust: nightly
       os: osx
+      before_script:
+        # TODO: https://github.com/rust-lang-nursery/futures-rs/pull/1685#issuecomment-506927847
+        - rustup update
 
     - name: cargo test
       rust: nightly
       os: linux
+      before_script:
+        # TODO: https://github.com/rust-lang-nursery/futures-rs/pull/1685#issuecomment-506927847
+        - rustup update
 
     - name: cargo build (with minimal versions)
       rust: nightly