.travis.yml: fix brew issue with libtool by uninstall / reinstall

brew uninstall libtool && brew install libtool

as per:

Homebrew/legacy-homebrew#43874

Signed-off-by: Michael Ira Krufky <mkrufky@linuxtv.org>

.travis.yml

@@ -242,5 +242,7 @@ script:
 before_install:
   - if [ "$TRAVIS_OS_NAME" != "linux" ]; then
         brew update;
+        brew uninstall libtool;
+        brew install libtool;
         brew install libdvbpsi libhdhomerun;
     fi