LibC++ without release branch.

script/ci_install.sh

     if [[ ! -d "${DEPENDENCY_DIR}/llvm-source" ]]
     then
         LLVM_RELEASE=release_50
-        git clone --depth=1 -b ${LLVM_RELEASE} https://github.com/llvm-mirror/llvm.git llvm-source
-        git clone --depth=1 -b ${LLVM_RELEASE} https://github.com/llvm-mirror/libcxx.git llvm-source/projects/libcxx
-        git clone --depth=1 -b ${LLVM_RELEASE} https://github.com/llvm-mirror/libcxxabi.git llvm-source/projects/libcxxabi
+        git clone --depth=1 https://github.com/llvm-mirror/llvm.git llvm-source
+        git clone --depth=1 https://github.com/llvm-mirror/libcxx.git llvm-source/projects/libcxx
+        git clone --depth=1 https://github.com/llvm-mirror/libcxxabi.git llvm-source/projects/libcxxabi
 
         if [[ -z "${BUILD_32_BITS}" ]]
         then