Work round homebrew error on travis.

See: https://github.com/matthew-brett/multibuild/pull/274#issuecomment-547314285

Thanks Matti.

tests/test_multibuild.sh

@@ -8,6 +8,11 @@ source tests/test_common_utils.sh
 source tests/test_fill_submodule.sh
 if [ -n "$IS_OSX" ]; then
     source osx_utils.sh
+
+    # To work round:
+    # https://travis-ci.community/t/syntax-error-unexpected-keyword-rescue-expecting-keyword-end-in-homebrew/5623
+    brew update
+
     get_macpython_environment $PYTHON_VERSION ${VENV:-""} $MB_PYTHON_OSX_VER
     source tests/test_python_install.sh
     source tests/test_fill_pyver.sh

travis_osx_steps.sh

@@ -20,6 +20,11 @@ source $MULTIBUILD_DIR/library_builders.sh
 function before_install {
     export CC=clang
     export CXX=clang++
+
+    # To work round:
+    # https://travis-ci.community/t/syntax-error-unexpected-keyword-rescue-expecting-keyword-end-in-homebrew/5623
+    brew update
+
     get_macpython_environment $MB_PYTHON_VERSION venv
     source venv/bin/activate
     pip install --upgrade pip wheel