Install a newer version of CMake (v3.8.2) (fixes #85).

.travis.yml

 dist: trusty
 sudo: required
 
+env:
+  global:
+    - DEPENDENCY_DIR="${TRAVIS_BUILD_DIR}/dependencies"
+
+cache:
+  directories:
+    - ${DEPENDENCY_DIR}
+
+
 matrix:
     include:
     - env: CXX=g++-7 CC=gcc-7
             - libc++-dev
           sources: *sources
 
+install:
+    - mkdir -p "${DEPENDENCY_DIR}" && cd "${DEPENDENCY_DIR}"
+    - |
+        if [[ ! -d "$(ls -A ${DEPENDENCY_DIR}/cmake)" ]]; then
+          CMAKE_URL="https://cmake.org/files/v3.6/cmake-3.8.2-Linux-x86_64.tar.gz"
+          mkdir -p cmake && travis_retry wget --no-check-certificate --quiet -O - "${CMAKE_URL}" | tar --strip-components=1 -xz -C cmake
+        fi
+        export PATH="${DEPENDENCY_DIR}/cmake/bin:${PATH}"
+
+
 script:
     - script/ci_build.sh