Use Clang 3.9 on trusty as Clang 3.6 on precise seems broken

.travis.yml

@@ -434,12 +434,11 @@ matrix:
       addons:
         apt:
           sources:
-            - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.6
+            - llvm-toolchain-trusty-3.9
           packages:
-            - clang-3.6
+            - clang-3.9
       env:
-        - MATRIX_EVAL="CC=clang-3.6 && CXX=clang++-3.6 && INFO=C++14-pure-release"
+        - MATRIX_EVAL="CC=clang-3.9 && CXX=clang++-3.9 && INFO=C++14-pure-release"
       script:
         - cmake --version
         - mkdir ./build_pure_14_release
@@ -452,12 +451,11 @@ matrix:
       addons:
         apt:
           sources:
-            - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.6
+            - llvm-toolchain-trusty-3.9
           packages:
-            - clang-3.6
+            - clang-3.9
       env:
-        - MATRIX_EVAL="CC=clang-3.6 && CXX=clang++-3.6 && INFO=C++14-pure-debug"
+        - MATRIX_EVAL="CC=clang-3.9 && CXX=clang++-3.9 && INFO=C++14-pure-debug"
       script:
         - cmake --version
         - mkdir ./build_pure_14_debug
@@ -470,12 +468,11 @@ matrix:
       addons:
         apt:
           sources:
-            - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.6
+            - llvm-toolchain-trusty-3.9
           packages:
-            - clang-3.6
+            - clang-3.9
       env:
-        - MATRIX_EVAL="CC=clang-3.6 && CXX=clang++-3.6 && INFO=C++14-avx-debug"
+        - MATRIX_EVAL="CC=clang-3.9 && CXX=clang++-3.9 && INFO=C++14-avx-debug"
       script:
         - cmake --version
         - mkdir ./build_avx_14_debug