fix(senna): force Yarn to respect "resolutions" field

Looks like we were running into a variant of:

    yarnpkg/yarn#5039

My "resolutions" weren't having the expected effect (we were getting
both 2.4.0 and 2.2.6 of "metal-uri"), even after deleting the yarn.lock
and recreating it.

So here, I add an empty "workspaces" config, like the other projects
under "maintenance" have (well, technically, they all have non-empty
"workspaces" config) and that makes it work. Now we only have 2.2.6 in
the lockfile!

Note that because I deleted the lockfile before preparing this commit,
it also includes some innocuous updates. The build still works.

maintenance/projects/senna/package.json

@@ -10,7 +10,7 @@
 		"metal-path-parser": "^1.0.3",
 		"metal-promise": "^2.0.1",
 		"metal-structs": "^1.0.0",
-		"metal-uri": "2.2.6",
+		"metal-uri": "^2.2.6",
 		"metal-useragent": "^3.0.0"
 	},
 	"description": "A blazing-fast Single Page Application engine",
@@ -68,5 +68,6 @@
 		"lint:fix": "cd ../../.. && yarn lint:fix",
 		"prepublishOnly": "gulp && npm run compile"
 	},
-	"version": "2.7.9"
+	"version": "2.7.9",
+	"workspaces": []
 }