Note Ruby version requirement in the gemspec

People may not be aware that we don't support Ruby 1.8.7; for example,
see:

  https://github.com/causes/scss-lint/issues/32

Let's make it explicit in the gemspec.

Change-Id: I1e601d902811768e171293d0e213045dcc2e4174
Reviewed-on: https://gerrit.causes.com/27597
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Noah Silas <noah@causes.com>
Remove Linter superclass from LinterRegistry

The superclass `Linter` was incorrectly being returned in the list of
registered linters. Remove the inclusion of the `LinterRegistry` module
to remove it.

This didn't previously have any negative effect because the `Linter`
class `run` method is effectively a no-op.

Change-Id: I1199f0b35ff16f693d3d17e99288cd93add4200b
Reviewed-on: https://gerrit.causes.com/27622
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Kim Douglas Clarence Mason <kim.mason@causes.com>
Add show-linters flag

Add `--show-linters` flag which prints out the list of all linters
registered with `scss-lint`.

Change-Id: I909d15f1e8bfc631d64af3822717833a7d6548c4
Reviewed-on: https://gerrit.causes.com/27623
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Kim Douglas Clarence Mason <kim.mason@causes.com>
Change ignore-linter option to use camel case names

Change the `--ignore-linter` option to use the camel case form of linter
names (e.g. `DeclaredNameLinter` instead of `declared_name_linter`).

Change-Id: Ic4994ed6834d51b1e8f760a110c842b608ea8898
Reviewed-on: https://gerrit.causes.com/27624
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Kim Douglas Clarence Mason <kim.mason@causes.com>