Temporarily disable clang 3.9

.travis.yml

@@ -63,22 +63,6 @@ matrix:
         - MATRIX_EVAL="CC=gcc-8 && CXX=g++-8"
       compiler: g++-8
 
-    - os: linux
-      before_install:
-        - sudo apt-get update
-        - sudo apt-get purge g++
-        - sudo apt-get install clang-3.9
-        - CC=clang-3.9 && CXX=clang++-3.9
-      addons:
-        apt:
-          sources:
-            - ubuntu-toolchain-r-test
-          packages:
-            - ninja-build
-            - xorg-dev 
-            - libglu1-mesa-dev
-      compiler: clang++-3.9
-
     - os: linux
       before_install:
         - sudo apt-get update