Switch to Perlbrew and Perl 5.16.3

As Jaryd Malbin and moollaza explained, we shouldn't be using Ubuntu's default version of Perl, 5.14.2, because it is not a version that DDG tests. DDG tests 5.10, 5.16 and 5.18. So this pull request switches the VM to Perl 5.16.3 by means of Perlbrew.
http://perlbrew.pl/