project: bump cocoapods version to 2.0.0

bump Cocoapods version

bump Cocoapods version

fix typo in Presentable

fix swifltint errors

Bump CocoaPods version

Imp/travis xcode9.3 test simulator11.3 (RxSwiftCommunity#52)

* travis.yml: update osx_image to xcode9.3

This commit updates the Travis CI osx_image the tests are running on and
so follows the path taken to migrate the project to Xcode 9.3 and
Swift 4.1

* travis.yml: update test iOS Simulator to 11.3

This commit updates the Simulator the tests are running on to the 11.3
iOS version. It will ensure the compatibility with the latest version.

* docs: update Jazzy documentation

fix: prevent a reentrancy issue

readme: update regarding the updates NextFlowItems

project: remove Carthage build phase

This commit removes the 'RxFlow project' Carthage custom build phase.
It appears that it led to rejections on AppStore submissions as there
were nested bundle.