Merge pull request #4266 from libgit2/ethomson/travis-explicit-openssl

travis: install openssl explicitly
diff --git a/script/install-deps-osx.sh b/script/install-deps-osx.sh
index 4b8393b..27d8096 100755
--- a/script/install-deps-osx.sh
+++ b/script/install-deps-osx.sh
@@ -5,4 +5,5 @@
 brew update
 brew install homebrew/dupes/zlib
 brew install curl
+brew install openssl
 brew install libssh2