.travis.yml: update to gcc 4.8

Author: davidmalcolm

.travis.yml

@@ -4,4 +4,4 @@ compiler:
 script: make
 before_install:
  - sudo apt-get update -qq
- - sudo apt-get install -qq gcc-4.6-plugin-dev python-six python-pygments graphviz python-lxml
+ - sudo apt-get install -qq gcc-4.8-plugin-dev python-six python-pygments graphviz python-lxml