ci: install homebrew's curl on mac

script/cibuild.sh

@@ -7,7 +7,7 @@ then
 fi
 
 if [ "$TRAVIS_OS_NAME" = "osx" ]; then
-    export PKG_CONFIG_PATH=$(ls -d /usr/local/Cellar/zlib/*/lib/pkgconfig | paste -s -d':' -)
+	export PKG_CONFIG_PATH=$(ls -d /usr/local/Cellar/{curl,zlib}/*/lib/pkgconfig | paste -s -d':' -)
 fi
 
 mkdir _build

script/install-deps-osx.sh

@@ -4,4 +4,5 @@ set -x
 
 brew update
 brew install homebrew/dupes/zlib
+brew install curl
 brew install libssh2