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".

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