Remove not understood --c++11 brew install option

For OSX travis builds:
Since recently, brew install --c++11 stops the build.
Error: invalid option: --c++11
The command "./.ci/install-deps.sh" failed and exited with 1 during .
Your build has been stopped.

.ci/install-deps.sh

@@ -3,7 +3,7 @@
 set -euo pipefail
 
 if [ "$TRAVIS_OS_NAME" == "osx" ]; then
-  brew install --c++11 \
+  brew install \
     pkg-config \
     aften \
     libsamplerate \