Add JavaScript rule for semi colons

While I personally prefer the "never" option for this rule, we haven't
discussed which guideline to follow, so for now I'm applying the rule
CoffeeScript used when generating these files.

.eslintrc.yml

@@ -49,6 +49,9 @@ rules:
     - error
     - double
     - avoidEscape: true
+  semi:
+    - error
+    - always
   semi-spacing: error
   space-before-blocks: error
   space-before-function-paren: