Fix scss_lint gem naming

circle.yml

@@ -5,7 +5,7 @@ machine:
 dependencies:
   pre:
     - pip install -e .[testing]
-    - gem install scss-lint
+    - gem install scss_lint
 
 test:
   override:

docs/contributing/css_guidelines.rst

@@ -35,13 +35,13 @@ automatically recompiling when any changes are observed, by running:
 Linting SCSS
 ~~~~~~~~~~~~
 
-Wagtail uses the "scss-lint" Ruby Gem for linting.
+Wagtail uses the "scss_lint" Ruby Gem for linting.
 
 Install it thus:
 
 .. code-block:: console
 
-    $ gem install scss-lint
+    $ gem install scss_lint
 
 
 Then run the linter from the wagtail project root: