travis: Install libtool for OS X builds

CI is failing due to missing libtoolize, so I'm trying this.

.travis.yml

@@ -8,7 +8,7 @@ language: c
 
 install:
   - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew update > /dev/null; fi
-  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew install openssl libidn rtmpdump libssh2 c-ares libmetalink libressl nghttp2; fi
+  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew install libtool openssl libidn rtmpdump libssh2 c-ares libmetalink libressl nghttp2; fi
 
 before_script:
   - ./buildconf