travis: workaround broken libtool #266

Newer Travis CI builds on OS X appear to have broken the installed
libtool.  Workaround it until things get fixed.

travis/main.sh

@@ -34,10 +34,14 @@ update_os() {
 	case ${TRAVIS_OS_NAME} in
 	osx)
 		v --fold="brew_update" brew update
+		# We have to hack this package due to bugs in Travis CI.  See:
+		# https://github.com/libgd/libgd/issues/266
+		# https://github.com/Homebrew/legacy-homebrew/issues/43874
+		v --fold="brew_clean" brew uninstall libtool
 		# These packages are already installed in Travis, so omit them or brew fails.
-		# autoconf automake libtool pkg-config cmake libpng jpeg libtiff
+		# autoconf automake pkg-config cmake libpng jpeg libtiff
 		v --fold="brew_install" brew install \
-			gettext freetype fontconfig webp xz
+			gettext freetype fontconfig libtool webp xz
 		;;
 	esac
 }