Update travis configuration
diff --git a/.travis.yml b/.travis.yml
index a5b3151..44570fd 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,12 +12,15 @@
       script:
         - cargo test
         - cargo build --features unstable
-        - cargo doc --no-deps
+        - RUSTFLAGS='--cfg procmacro2_unstable' cargo test
+        - RUSTFLAGS='--cfg procmacro2_unstable' cargo build --features unstable
+        - RUSTFLAGS='--cfg procmacro2_unstable' cargo doc --no-deps
       after_success:
         - travis-cargo --only nightly doc-upload
 
 script:
   - cargo test
+  - RUSTFLAGS='--cfg procmacro2_unstable' cargo test
 env:
   global:
     - TRAVIS_CARGO_NIGHTLY_FEATURE=""