Use --fuzzy when running casher and deploy

This can prevent unpleasant situation where the version of Ruby that
RVM considers most up-to-date is not present and the deploy fails.

An example of such situation is described in https://github.com/travis-ci/travis-ci/issues/2202
Add note on how specs run [skip ci]
--use-mirrors is deprecated.
Merge pull request #233 from ifduyue/use-mirrors-is-deprecated

--use-mirrors is deprecated.
changed an if to an elif so we don't run a command twice
ruby: allow selecting rubies with the `ruby` key

This also adds chruby support, although this is a beta feature for now.
ruby: download chruby from gist

This wil make it easier to change things without actually having to
redeploy the worker all the time.
ruby: always get latest version of gist
Ignore non-existent addons.

Fixes travis-ci/travis-ci#2063.
Stub chruby for tests.
Fold potentially noisy RVM fetching
Merge pull request #237 from bjeanes/patch-1

Fold potentially noisy RVM fetching
Merge branch 'master' into ha-feature-windows