Build docs on beta Rust because rustc_macro breaks

This is vaguely similar to
rust-lang/cargo#3132 and
rust-lang/rust#36820, except it just started
again today, and those issues were closed several days ago.

We don't do this on stable, because we have multiple stable platforms
and we need them to build fast.

.travis.yml

@@ -44,7 +44,7 @@ script:
   travis-cargo bench $EXTRA_ARGS &&
   travis-cargo doc $EXTRA_ARGS
 after_success:
-- travis-cargo --only nightly doc-upload
+- travis-cargo --only beta doc-upload
 before_deploy: "./build-release cage ${TRAVIS_TAG}-${TRAVIS_OS_NAME}"
 env:
   global: