Use gcc 4.7 with travis.

gcc 4.6 (the default) does have some C++11 support, but its flag is -std=c++0x. Since 4.7 was released in 2012, I think it's reasonable to require it.

.travis.yml

@@ -2,6 +2,11 @@ language: c++
 compiler:
   - clang
   - gcc
+before_install:
+  - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
+  - sudo apt-get update -qq
+  - if [ "$CXX" = "g++" ]; then sudo apt-get install -qq g++-4.7; fi
+  - if [ "$CXX" = "g++" ]; then export CXX="g++-4.8" CC="gcc-4.8"; fi
 before_script:
   - mkdir build
   - cd build