Update reference to Ubuntu 18.04

Leaving this more vague since there are now multiple base image options available.

docs/specs.md

@@ -3,9 +3,9 @@
 Autograders are uploaded to Gradescope in zip format. The file must
 contain at least two files in the root of the archive:
 
-- **setup.sh**: a setup (Bash) script that installs all your dependencies. We're
-  running on Ubuntu 18.04 images, so you can use apt, or any other means of
-  setting up packages.
+- **setup.sh**: a setup (Bash) script that installs all your dependencies.
+  By default, we're running on Ubuntu images, so you can use apt, or any
+  other available means of setting up packages.
 
 - **run_autograder**: an executable script, in any language (with
   appropriate `#!` line), that compiles and runs your autograder suite