travis: install the Perl deps in the Perl dir

As the rrdtool ./configure said :

	By default the rrdtool perl modules are installed together with rrdtool
	in $prefix/lib/perl. You have to put a 'use lib qw($prefix/lib/perl)'
	into your scripts when you want to use them. When you set this option
	the perl modules will get installed wherever your perl setup thinks it
	is best.