update travis (nw)

.travis.yml

@@ -1,10 +1,22 @@
 language: cpp
+addons:
+    apt:
+        sources:
+            - llvm-toolchain-precise
+            - ubuntu-toolchain-r-test
+        packages:
+            - clang-3.7
+            - g++-5
+            - gcc-5
 compiler:
   - gcc
   - clang
 os:
   - linux
   - osx
+install:
+    - if [ "$CXX" = "g++" ]; then export CXX="g++-5" CC="gcc-5"; fi
+    - if [ "$CXX" = "clang++" ]; then export CXX="clang++-3.7" CC="clang-3.7"; fi
 matrix:
   exclude:
     - os: osx