Prepare travis-ci.org integration, fix build on Ubuntu 12.04 LTS

[Travis CI](http://travis-ci.org) is a free hosted continuous integration platform for open-source projects.  It allows automated testing for GitHub-hosted projects.  I've enabled its use in my (to be obsoleted) [fork](https://github.com/pah/rapidjson).

After adding the configuration and the additionally required fix, the current master branch build successfully on 64-bit clang and gcc, see https://travis-ci.org/pah/rapidjson.  By default, new pull-requests are automatically built as well.