build: update brew

scripts/bootstrap-depends.sh

@@ -79,6 +79,7 @@ PIP_PKGS="${TRAVIS_BUILD_DIR}/tests/pydnstest/requirements.txt cpp-coveralls"
 if [ "${TRAVIS_OS_NAME}" == "osx" ]; then
 	DEPEND_CACHE="https://dl.dropboxusercontent.com/u/2255176/resolver-${TRAVIS_OS_NAME}-cache.tar.gz"
 	curl "${DEPEND_CACHE}" > cache.tar.gz && tar -xz -C ${HOME} -f cache.tar.gz || true
+	brew update
 	brew install --force makedepend python libtasn1 || true
 	brew link --overwrite python || true
 	pip install --upgrade pip || true