travis: fix OSX build by re-installing libtool

Apparently due to a broken homebrew install

fixes #934
Closes #939

.travis.yml

@@ -8,6 +8,7 @@ language: c
 
 install:
   - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew update > /dev/null; fi
+  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew reinstall -s libtool > /dev/null; fi
   - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew install openssl libidn rtmpdump libssh2 c-ares libmetalink libressl nghttp2; fi
 
 before_script: