Gracefully handle unexpected source structure

Closes https://github.com/CocoaPods/Core/issues/110
Make unexpected source structure `Informative` even more informative
Merge branch 'master' into feature-non-version-dir-name

* master:
  [travis-ci] Correctly set the LANG environment
  Trying to fix Travis build error with 'invalid byte sequence'
  Fix .travis.yml to add en_US.UTF8 locale
  Fixing Rakefile exception cases
  Fixing Rakefile's bootstrap
  [Specification::DSL] Introduce default_subspecs
  Update docs in DSL for podspec attribute.
  Fix wrong number on an issue reference in the changelog
  [DSL] Don't prepend the spec name to default_subspec
  Changelog and fixing a quote
  [Rubocop] Adding exclusion of `spec/fixtures/**` in the `.rubocop-cocoapods.yml` common rules config file even if it has to be repeated in the .rubocop.yml config file if we need to add other exclusions (like `lib/*/vendor/**`)
  [Rubocop] trying to split CocoaPods/Core specific rubocop rules and CP-common rules, so that we start using a common .rubocop-cocoapods.yml across all CP repos See discussion in CocoaPods/cocoapods-plugins#17
  Added missing space to acceptor
  Add labels to all the linter warnings and errors
  [Gemspec] Drop requirement for ActiveSupport < 4
  Simplify URL validation.
  Validate URIs before making requests, only catch SocketErrors.

Conflicts:
	CHANGELOG.md
Merge branch 'feature-non-version-dir-name'

* feature-non-version-dir-name:
  Make unexpected source structure `Informative` even more informative
  Gracefully handle unexpected source structure