Fix theme serve failing with the --host property

[karreiro]

Fix theme serve failing with the --host property (#1765) by renaming the parameters from http_bind to host.

WHY are these changes introduced?

This was failing because we were calling DevServer#start in the theme/commands/serve.rb file with the **flags and the names were different.

WHAT is this pull request doing?

This PR renames parameters from http_bind to host.

How to test your changes?

shopify theme serve --host=0.0.0.0

Here's the previous behavior:

Here's how it actual behavior (with this PR applied):

Update checklist

• I've added a CHANGELOG entry for this PR (if the change is public-facing)
• I've considered possible cross-platform impacts (Mac, Linux, Windows).
• I've left the version number as is (we'll handle incrementing this when releasing).
• I've included any post-release steps in the section above.

[macournoyer]

Nice catch! 👏