Build on CentOS 6

Hopefully a solution to #102.

To check:

    $ git clone https://github.com/aanand/fig.git
    $ cd fig
    $ git checkout build-on-centos6
    $ script/build-linux
    $ docker run -v `pwd`/dist/fig:/usr/bin/fig -i -t centos:centos6 fig --version

And if CentOS 6 is your host OS:

    $ fig --version