travis: replace use of deprecated homebrew/dupes tap
The formulae provided by the homebrew/dupes tap are deprecated since at
least April 4, 2017, with formulae having been migrated to
homebrew/core.
Replace the deprecated reference to "homebrew/dupes/zlib" with only
"zlib".
diff --git a/script/install-deps-osx.sh b/script/install-deps-osx.sh
index 27d8096..94314db 100755
--- a/script/install-deps-osx.sh
+++ b/script/install-deps-osx.sh
@@ -3,7 +3,7 @@
set -x
brew update
-brew install homebrew/dupes/zlib
+brew install zlib
brew install curl
brew install openssl
brew install libssh2
