Fixing compiler package for clang

Due to travis having moved to xenial, some builds are failing (as they are using the wrong toolchain). This attempts to fix.

Author: etr

.travis.yml

@@ -288,8 +288,6 @@ matrix:
     - os: linux
       addons:
         apt:
-          sources:
-            - llvm-toolchain-trusty-4.0
           packages:
             - clang-4.0
       env:
@@ -298,18 +296,13 @@ matrix:
     - 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"
     - os: linux
       addons:
         apt:
-          sources:
-            - llvm-toolchain-trusty-6.0
-            - ubuntu-toolchain-r-test
           packages:
             - clang-6.0
       env:
@@ -318,12 +311,22 @@ matrix:
       addons:
         apt:
           sources:
-            - llvm-toolchain-trusty-7
+            - llvm-toolchain-xenial-7
             - ubuntu-toolchain-r-test
           packages:
             - clang-7
       env:
         - MATRIX_EVAL="CC=clang-7 && CXX=clang++-7"
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - llvm-toolchain-xenial-8
+            - ubuntu-toolchain-r-test
+          packages:
+            - clang-8
+      env:
+        - MATRIX_EVAL="CC=clang-8 && CXX=clang++-8"
     - os: linux
       compiler: clang
       addons: