Update circle.yml

circle.yml

@@ -8,9 +8,10 @@ dependencies:
   cache_directories:
     - ~/cmake-3.11.0-Linux-x86_64
   pre:
-    - wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add -
-    - sudo apt-get install clang-5.0 lldb-5.0
-    - export CXX=clang-5.0
+    - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
+    - sudo apt-get update
+    - sudo apt-get install gcc-6 g++-6
+    - sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 20 --slave /usr/bin/g++ g++ /usr/bin/g++-6
   override:
     - >
       if [ ! -d ~/cmake-3.11.0-Linux-x86_64 ]; then