Install cmake only if needed

Author: dinhvh

scripts/travis/before-script.sh

@@ -5,7 +5,9 @@ if test x"`uname`" = xDarwin ; then
   sudo systemsetup -settimezone America/Los_Angeles
   brew update || brew update || :
   brew uninstall xctool && brew install --HEAD xctool
-  brew install cmake
+  if ! which cmake >/dev/null ; then
+    brew install cmake
+  fi
 elif test x"`uname`" = xLinux ; then
   git clone --depth=1 https://github.com/dinhviethoa/libetpan
   cd libetpan