(GH-51) [module/forge] Always expand expected_version

Forge modules can have a version of `:latest`, which will set the
expected version based on the latest version of a module on the forge.
However this means that the variable always needs to be expanded if it's
latest, and there were instances where the value was not expanded.
This commit resolves the issue by wrapping all uses of `@expected_version` in
a helper method that will lazily expand the value on demand.
[Gemfile] Add rspec-system to gemfile
[spec/system] Stub out basic rspec-system tests
[spec/system] forge module coverage
[.rspec] Change default path to spec/unit

rspec-system tests are very heavyweight and it's annoying when they are
run when one only wants to run unit tests. This commit adds a .rspec
file that changes the default path to spec/unit
[travis] Don't install rspec-system tools on travis-ci
Merge branch 'testing/next/rspec-system' into next