Set modern compiler for travis

.travis.yml

@@ -2,9 +2,34 @@ sudo: required
 dist: trusty
 language: cpp
 cache: ccache
-compiler:
-  - clang
-  - gcc
+matrix:
+  include:
+    - compiler: gcc
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-5
+      env: COMPILER=g++-5
+    - compiler: clang
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-precise-3.6
+          packages:
+            - clang-3.6
+      env: COMPILER=clang++-3.6
+    - compiler: clang
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-precise-3.7
+          packages:
+            - clang-3.7
+      env: COMPILER=clang++-3.7
 script:
   - mkdir build
   - cd build
@@ -21,12 +46,8 @@ addons:
   apt:
     sources:
       - ubuntu-toolchain-r-test
-      - llvm-toolchain-precise-3.7
     packages:
       - cmake
       - cmake-data
-      - gcc-5
-      - g++-5
-      - clang-3.7