Fixes #2752.

Update for https://github.com/CocoaPods/CocoaPods/commit/7840787e05afb1da24bcfafa4a62f0de41ebf07a.

Revert "[Analyzer] Allow using CocoaPods with only externally-sourced dependencies in a   `Podfile` without specifying a source or displaying a warning"

This reverts commit 2cc9f01d.

Follow-up to https://github.com/CocoaPods/CocoaPods/commit/7840787e05afb1da24bcfafa4a62f0de41ebf07a.

Conflicts:
	lib/cocoapods/installer/analyzer.rb

Update for https://github.com/CocoaPods/CocoaPods/commit/3a61c4a265d97c41bff17a2c3aeea878f228041b.

[Analyzer] Improve handling of different types of duplicate dependencies

Closes https://github.com/CocoaPods/CocoaPods/issues/2753.

After much deliberation, the conclusion is that there is no good
techincal reason to enforce this, after all, and as such the decision
to deprecate it has been reverted.

https://github.com/travis-ci/travis-ci/issues/2918

[Analyzer] Allow using CocoaPods with only externally-sourced dependencies in a   `Podfile` without specifying a source or displaying a warning

Closes https://github.com/CocoaPods/CocoaPods/issues/2735.

[Sandbox] Symlink vendored framework headers into the public headers dir.

Fixes #1992.

For #1249

Improved sanitizing of configuration names which have a number prefix.

[Gemfile] Remove libxml2

This has no longer been a dependency of Xcodeproj