Merge branch 'release/0.71' into develop
Merge branch 'master' into develop
fix the case when PERL5LIB should be unset

ref: https://github.com/gugod/App-perlbrew/issues/377
Updated do_install_this() to unlink the .version file if it exists.  This
allows installed_perls() to recognize that do_install_this() changed the
version.

ref: https://github.com/gugod/App-perlbrew/issues/410