add ability to use the library as cmake subdirectory #9

.travis.yml

@@ -66,5 +66,16 @@ matrix:
       after_success: ./scripts/upload_coverage.sh
 before_install:
   - eval "${MATRIX_EVAL}"
+  - if [ "$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
 script:
   - ./scripts/build_all.sh