use a stable cmake now

.travis.yml

@@ -91,7 +91,7 @@ install:
   - mkdir -p ${DEPS_DIR} && cd ${DEPS_DIR}
   - |
     if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
-      CMAKE_URL="https://cmake.org/files/v3.13/cmake-3.13.0-rc3-Linux-x86_64.tar.gz"
+      CMAKE_URL="https://cmake.org/files/v3.12/cmake-3.12.4-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}
       echo ${PATH}