fix issues with travis-ci and perls 5.10 & 5.12

[joshrabinowitz]

Travis-ci build system can't find the tarballs for perl 5.10

in https://travis-ci.org/github/PerlDancer/Dancer/jobs/734108574 , see

Downloading archive: https://storage.googleapis.com/travis-ci-language-archives/perl/binaries/ubuntu/16.04/x86_64/perl-5.10.tar.bz2
curl: (22) The requested URL returned error: 404 Not Found
$ sudo tar xjf perl-5.10.tar.bz2 --directory /
tar (child): perl-5.10.tar.bz2: Cannot open: No such file or directory
tar (child): Error is not recoverable: exiting now
tar: Child returned status 2
tar: Error is not recoverable: exiting now
rm: cannot remove 'perl-5.10.tar.bz2': No such file or directory

Same problem for perl 5.12 at
https://travis-ci.org/github/PerlDancer/Dancer/jobs/734108573

[bigpresh]

Looks like the versions supported by Travis-CI these days are:

perlbrew list
   5.26                  (5.26.3)            (installed on Mon Mar 25 16:41:45 2019)
   5.26.3                                    (installed on Mon Mar 25 16:41:45 2019)
   5.26-extras           (5.26.2)            (installed on Mon Mar 25 16:45:42 2019)
   5.26-shrplib          (5.26.2)            (installed on Mon Mar 25 16:45:42 2019)
   5.26.2                                    (installed on Mon Mar 25 16:41:51 2019)
   5.24                  (5.24.3)            (installed on Mon Mar 25 16:46:12 2019)
   5.24.3                                    (installed on Mon Mar 25 16:46:12 2019)
   5.24-extras           (5.24.0)            (installed on Mon Mar 25 16:50:00 2019)
   5.24-shrplib          (5.24.0)            (installed on Mon Mar 25 16:50:00 2019)

So we'd need to either build our own perl first, or just target 5.24. I'd prefer to be able to test against older perls, but as Dancer1 isn't under particularly active development, it's not a massive deal.

Could even look in to changing from Travis to using GitHub Actions, but may be more work than it's worth.

[joshrabinowitz]

I think it's ok if we stop testing Dancer on perl 5.10 and 5.12.