Fix for #934
Same as neovim/homebrew-neovim#167
Related to broken brew.
Solution - reinstall libtool
fix for #934
@snikulov, thanks for your PR! By analyzing the annotation information on this pull request, we identified @sieben, @Jactry and @jay to be potential reviewers
travis: fix OSX build by re-installing libtool
Apparently due to a broken homebrew install