Remove semver exempt testing from 1.15 and stable builds

This feature tracks whatever is in nightly, so we reserve the right to
remain broken on compilers that are not nightly.

Author: dtolnay

.travis.yml

@@ -6,7 +6,6 @@ matrix:
     - rust: 1.15.0
     - rust: stable
     - rust: beta
-      script: cargo test
     - rust: nightly
       install: rustup target add wasm32-unknown-unknown
       script: cargo test --target wasm32-unknown-unknown --no-run
@@ -24,7 +23,6 @@ before_script:
 
 script:
   - cargo test
-  - RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo test
 
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