Add `-Duseshrplib` flag for `perlbrew`

Addresses travis-ci/travis-ci#1752
Merge branch 'master' into ha-bug-gh_ci_1752_perlbrew_flag
Provide shrplib versions for Perl 5.20 and 5.18

Instead of replacing all Perl runtimes with '-Duseshrplib' enabled,
provide additional runtimes with this feature selectively.
Merge pull request #251 from travis-ci/ha-bug-gh_ci_1752_perlbrew_flag

Add `-Duseshrplib` flag for `perlbrew`