[Podspec DSL] Document sha1 and sha256 hash verification
[Podspec DSL] HTTP source may have type and sha1, sha256

Fixes CocoaPods#2692
[Source Aggregate] Use getter instead of relying on ivar

The ivar is only populated due to a side-effect of getting it earlier in
the method. If it was stubbed this causes issues because the ivar isn't
the same as before.
Release 0.34.3
[Source Aggregate] Use select not keep_if

select returns a new array and keep_if isn't available in old ruby
Release 0.34.4
[Dependency] Add a cached prerelease? accessor
[Set] Remove unused resolver-related attributes/method
[Specification] Create subspec dependencies with a version requirement of self.version

@kylef this is one performance optimization for the resolver