external: update backward-cpp to v1.4

rename the directory name to backward-cpp

Closes: #803
Signed-off-by: Shengjing Zhu <i@zhsj.me>

external/backtrace-cpp/.travis.yml

@@ -1,25 +0,0 @@
-language: cpp
-compiler:
-  - gcc
-  - clang
-
-addons:
-  apt:
-    packages:
-      - valgrind
-
-install:
-  - DEPS_DIR="${TRAVIS_BUILD_DIR}/deps"
-  - mkdir ${DEPS_DIR} && cd ${DEPS_DIR}
-  - CMAKE_URL="http://www.cmake.org/files/v3.3/cmake-3.3.2-Linux-x86_64.tar.gz"
-  - mkdir cmake && travis_retry wget --no-check-certificate --quiet -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake
-  - export PATH=${DEPS_DIR}/cmake/bin:${PATH}
-  - pip install --user conan && export PATH=$PATH:$HOME/.local/bin
-  - cd ${TRAVIS_BUILD_DIR}
-  - mkdir build && cd build
-  - cmake .. -DBACKWARD_TESTS=ON
-  - cmake --build .
-
-script:
-  - valgrind ctest .. --verbose
-  - cd ${TRAVIS_BUILD_DIR} && conan test_package --build=outdated