InstallationError: don't search issues on < 1.8.6

open-uri on Ruby pre-1.8.6 simply will not open https URLs at all,
making it impossible to check the Github API using the same method
as Homebrew.

This may only be disabled temporarily until the issue search is
rewritten, for example to use the curl helper.
Refactor Requirement.expand
Allow explicit conversion of requirements to deps
Revert "Remove default_formula from MPIDependency."

This reverts commit 9aaa89dbe832387249ab3ea4e22a8018cc9fe30a.