fixed malformed if statement

contrib/gitflow-installer.sh

@@ -50,7 +50,7 @@ case "$1" in
 		;;
 	*)
 		echo "Installing git-flow to $INSTALL_PREFIX"
-		if [[ -d "$REPO_NAME" && -d "$REPO_NAME/.git" ]] ; then
+		if [ -d "$REPO_NAME" -a -d "$REPO_NAME/.git" ] ; then
 			echo "Using existing repo: $REPO_NAME"
 		else
 			echo "Cloning repo from GitHub to $REPO_NAME"