libzdb: use recommended deps
PostgresqlDependency: use canonical name for default formula
Requirement: fix typo
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.
Merge remote-tracking branch 'mxcl/master'