Fix macOS TravisCI builds

[jdm]

This change is 

[asajeffrey]

@bors-servo r+

[bors-servo]

📌 Commit 980d994 has been approved by asajeffrey

[bors-servo]

⌛ Testing commit 980d994 with merge 6e3c3ec...

[bors-servo]

💔 Test failed - checks-travis

[asajeffrey]

Oh what fun, if the llvm download fails, then homebrew tries building it from source, which unsurprisingly fails with a timeout after 10min.

==> Installing llvm
==> Downloading https://homebrew.bintray.com/bottles/llvm-8.0.0_1.high_sierra.bo
==> Downloading from https://akamai.bintray.com/ef/efce74f9e72bc5f894e40daccd3b7
########################################                                  56.6%
curl: (56) LibreSSL SSL_read: SSL_ERROR_SYSCALL, errno 60
Error: Failed to download resource "llvm"
Download failed: https://homebrew.bintray.com/bottles/llvm-8.0.0_1.high_sierra.bottle.tar.gz
Warning: Bottle installation failed: building from source.
==> Installing dependencies for llvm: cmake
==> Installing llvm dependency: cmake
==> Downloading https://homebrew.bintray.com/bottles/cmake-3.14.5.high_sierra.bo
==> Downloading from https://akamai.bintray.com/30/301e78f6aad3a0072abb17ca06616
######################################################################## 100.0%
==> Pouring cmake-3.14.5.high_sierra.bottle.tar.gz
==> Caveats
Emacs Lisp files have been installed to:
  /usr/local/share/emacs/site-lisp/cmake
==> Summary
🍺  /usr/local/Cellar/cmake/3.14.5: 5,674 files, 53MB
==> Downloading https://releases.llvm.org/8.0.0/llvm-8.0.0.src.tar.xz
######################################################################## 100.0%
==> Downloading https://releases.llvm.org/8.0.0/cfe-8.0.0.src.tar.xz
######################################################################## 100.0%
==> Downloading https://releases.llvm.org/8.0.0/clang-tools-extra-8.0.0.src.tar.
######################################################################## 100.0%
==> Downloading https://releases.llvm.org/8.0.0/openmp-8.0.0.src.tar.xz
######################################################################## 100.0%
==> Downloading https://releases.llvm.org/8.0.0/libcxx-8.0.0.src.tar.xz
######################################################################## 100.0%
==> Downloading https://releases.llvm.org/8.0.0/libunwind-8.0.0.src.tar.xz
######################################################################## 100.0%
==> Downloading https://releases.llvm.org/8.0.0/lld-8.0.0.src.tar.xz
######################################################################## 100.0%
==> Downloading https://releases.llvm.org/8.0.0/lldb-8.0.0.src.tar.xz
######################################################################## 100.0%
==> Downloading https://releases.llvm.org/8.0.0/polly-8.0.0.src.tar.xz
######################################################################## 100.0%
==> Downloading https://releases.llvm.org/8.0.0/compiler-rt-8.0.0.src.tar.xz
######################################################################## 100.0%
==> cmake -G Unix Makefiles .. -DCMAKE_C_FLAGS_RELEASE=-DNDEBUG -DCMAKE_CXX_FLAG
==> make
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

[bors-servo]

☀️ Test successful - checks-travis, status-appveyor
Approved by: asajeffrey
Pushing 6e3c3ec to master...