update travis cmake

.travis.yml

@@ -64,7 +64,17 @@ matrix:
       compiler: clang
 before_install:
   - eval "${MATRIX_EVAL}"
-  - if [ "$TRAVIS_OS_NAME" == 'osx' ]; then brew update; brew install git-lfs; fi
+  - if [ "$TRAVIS_OS_NAME" == 'osx' ]; then
+    brew update;
+    brew upgrade cmake;
+    brew install git-lfs;
+    fi
+  - if [ "$TRAVIS_OS_NAME" == 'linux' ]; then
+    mkdir $HOME/cmake;
+    export PATH="$HOME/cmake/bin:$PATH";
+    travis_retry wget -q https://cmake.org/files/v3.11/cmake-3.11.4-Linux-x86_64.sh;
+    sh cmake-3.11.4-Linux-x86_64.sh --prefix=$HOME/cmake --exclude-subdir --skip-license;
+    fi
 before_script:
   - git submodule update --init --recursive
   - git lfs install