Use perl explicitly

Hi. ```bin/build-dist``` and ```bin/cpan-install``` seem to have used a different perl from the one specified by [```TRAVIS_PERL_VERSION```](http://docs.travis-ci.com/user/languages/perl/#Environment-Variable), and thus, installing modules required to configure before ```build-dist``` didn't work as expected. This PR is to fix it.

A sample project and ```.travis.yml``` that I used to test is found at https://github.com/charsbar/sandbox . (The Makefile.PL there is actually a mock for the one in DBD::SQLite repository).