Release 0.17.1
[Linter] Enable warning about FileList
[Dependency] Raise if version requirements are specified for an external
source.

Closes #787
[Docs] Minor clarifications to the Specification DSL
[Specification] Raise informative if unable to find a subspec
[TargetDefinition] Don't returns subspec deps with podspec directive.

Closes CocoaPods/CocoaPods#928
[Specs] Fix for recent changes
Release 0.17.2