Merge branch 'release/0.70' into develop
make it sh friendly. fix GH #407
fix PERL5LIB leftover bug.

Bug description:

    Given PERL5LIB is empty, run:

        perlbrew use @foo
        perlbrew use @bar

    Now PERL5LIB contains the lib path for both @foo and @bar.
Merge branch 'feature/local-lib-cleanup' into develop
rebuild
fix a PERL5LIB-leftover bug

To reproduce the bug:

    Given PERL5LIB is blank, run:

        perlbrew use @foo
        perlbrew off

    PERL5LIB still contains the paths to @foo, which is wrong.
always catch up with CPAN::Perl::Releases
rebuild
require local::lib 2.000014
avoid omitting a "unset VAR" command when the VAR is already blank/unset.
version up.
Merge branch 'release/0.71'