[Specs] Fix spec helper
[Gemspec] Specify Ruby version
[Installer] Call the specification hooks per each target definition

Closes #985
[Travis] Install Mercurial
[Travis] Install Mercurial (link)
[Travis] Install Mercurial (sudo)
[Travis] Install Mercurial (chown)
[Installer] Improve specs for post installhooks

See #985
[Specs] Try to fix Coveralls issues
[ExternalSources] Refine rename of the :local option to :path

See #971