[Sources] Use `git pull` with the `--ff-only` flag

This fixes issue where since we were using `--no-commit`, git was
starting a merge but not committing it. Then subsequent `git pull`'s would
result in the following error from git:

> You have not concluded your merge (MERGE_HEAD exists).
> Please, commit your changes before you can merge.

This commit will mean that `--ff-only` is passed to `git pull`  which means
it wont rebase or merge. It will only allow a fast forward to progress
with the pull.

Fixes #2024

I'm sure everyone is going to hate me for doing this.

Fixes #1771
Closes #1966

Try to detect and remove local paths from error messages when generating...

Print current version when the repo/lockfile requires a higher version

* segiddins-warn-for-deprecated-specs-on-install:
  [InstallerSpec] Check for deprecation message when spec.deprecated = true
  Warn for deprecated specs on install

Merge branch 'warn-for-deprecated-specs-on-install' of https://github.com/segiddins/CocoaPods into segiddins-warn-for-deprecated-specs-on-install

* 'warn-for-deprecated-specs-on-install' of https://github.com/segiddins/CocoaPods:
  [InstallerSpec] Check for deprecation message when spec.deprecated = true
  Warn for deprecated specs on install

* segiddins-warn-for-deprecated-specs-on-outdated:
  Update Changelog
  Refactored Command::Outdated to make it stub-able
  Warn for deprecated specs on outdated

Merge branch 'warn-for-deprecated-specs-on-outdated' of https://github.com/segiddins/CocoaPods into segiddins-warn-for-deprecated-specs-on-outdated

* 'warn-for-deprecated-specs-on-outdated' of https://github.com/segiddins/CocoaPods:
  Update Changelog
  Refactored Command::Outdated to make it stub-able
  Warn for deprecated specs on outdated

Conflicts:
	CHANGELOG.md

* segiddins-check-if-sudo-needed-for-gem-install:
  Conditionally include `sudo` in suggested update command

Merge branch 'check-if-sudo-needed-for-gem-install' of https://github.com/segiddins/CocoaPods into segiddins-check-if-sudo-needed-for-gem-install

* 'check-if-sudo-needed-for-gem-install' of https://github.com/segiddins/CocoaPods:
  Conditionally include `sudo` in suggested update command

Conflicts:
	spec/unit/sources_manager_spec.rb

Closes https://github.com/CocoaPods/CocoaPods/issues/2049

Helpful exception when reading version information with merge conflict

Improved error message when pulling a spec repo raises

Closes https://github.com/CocoaPods/CocoaPods/issues/2092

Move repo update to install section of travis.yml

Needed so that the test that depend on Core work (re HTTP requests)

As / points to the root of the domain, the expected result was actually wrong.

Try to detect and remove local paths from error messages when generating a ‘search for existing tickets’ URL.

Closes https://github.com/CocoaPods/CocoaPods/issues/2057