use container-based infrastructure

.travis.yml

@@ -1,34 +1,50 @@
 language: cpp
+sudo: false
 
-env:
 matrix:
-    - COMPILER=g++-4.9
-      COMPILER_PACKAGE=g++-4.9
-
-    - COMPILER=g++-4.8
-      COMPILER_PACKAGE=g++-4.8
-
-    - COMPILER=g++-4.7
-      COMPILER_PACKAGE=g++-4.7
-
-    - COMPILER=clang++-3.5
-      COMPILER_PACKAGE=clang-3.5
-
-    - COMPILER=clang++-3.6
-      COMPILER_PACKAGE=clang-3.6
-
+  include:
+    - env: COMPILER=g++-4.9
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-     - boost-latest
+          packages:
+            - g++-4.9
+    - env: COMPILER=g++-4.8
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.8
+    - env: COMPILER=g++-4.7
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-4.7
+    - env: COMPILER=clang++-3.5
+      addons:
+        apt:
+          sources:
             - llvm-toolchain-precise-3.5
+          packages:
+            - clang-3.5
+    - env: COMPILER=clang++-3.6
+      addons:
+        apt:
+          sources:
             - llvm-toolchain-precise-3.6
-     - llvm-toolchain-precise-3.7
+          packages:
+            - clang-3.6
+
+addons:
+  apt:
+    sources:
+     - boost-latest
      - george-edison55-precise-backports
     packages:
-     - ${COMPILER_PACKAGE}
-     - build-essential
      - cmake
      - cmake-data
      - libboost1.55-dev