Enable the use of cpanfile mirror syntax

[kiwiroy]

This adds the

ability to specify mirrors using cpanfile's mirror syntax.
comment

which is in the DSL also mentioned here

carton commands install and update will use the mirror specified in cpanfile, as below, in the same way the PERL_CARTON_MIRROR environment variable acts, but permits easier source control of the setting.

mirror "$url";

Although there is this alternate syntax which is more specific, it is more verbose and utilising the cpanfile scoped mirror is more attractive.

carton/lib/Carton.pm

Lines 100 to 103 in d7fcb61

	# local mirror (darkpan)
	requires 'Plack', '== 0.9981',
	dist => 'MYCOMPANY/Plack-0.9981-p1.tar.gz',
	mirror => 'https://pause.local/';

Travis fails on perl < 5.14 due to requiring dist: trusty as per #268