travis: don't install CMake on OS X Homebrew will error out because it's already installed.
diff --git a/script/install-deps-osx.sh b/script/install-deps-osx.sh
index a0216e9..5510379 100755
--- a/script/install-deps-osx.sh
+++ b/script/install-deps-osx.sh
@@ -3,4 +3,4 @@
set -x
brew update
-brew install libssh2 cmake
+brew install libssh2