travis ci, try 4

.travis.yml

@@ -1,17 +1,18 @@
-language: cpp
-
-compiler: clang
-
-os: linux
-addons:
-  apt:
-    sources:
-      - ubuntu-toolchain-r-test
-    packages:
-      - clang-6.0
-      - clang
+matrix:
+  include:
+    # works on Trusty
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - llvm-toolchain-trusty-5.0
+          packages:
+            - clang-5.0
+      env:
+        - MATRIX_EVAL="CC=clang-5.0 && CXX=clang++-5.0"
 
 before_install:
+ - eval "${MATRIX_EVAL}"
  - sudo add-apt-repository --yes ppa:ubuntu-sdk-team/ppa
  - sudo apt-get update -qq
  - sudo apt-get install qtbase5-dev