Bump aetna from 1.0.0-alpha.17 to 1.0.0-alpha.20 (#148)

Bumps [aetna](https://github.com/pressbooks/aetna) from 1.0.0-alpha.17 to 1.0.0-alpha.20.
- [Release notes](https://github.com/pressbooks/aetna/releases)
- [Commits](https://github.com/pressbooks/aetna/commits)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -59,7 +59,7 @@
 		"pressbooks-build-tools": "^0.13.0"
 	},
 	"dependencies": {
-		"aetna": "^1.0.0-alpha.17",
+		"aetna": "^1.0.0-alpha.20",
 		"isotope-layout": "^3.0.6",
 		"jquery-bridget": "^2.0.1",
 		"js-cookie": "^2.2.0",

yarn.lock

@@ -285,9 +285,9 @@ adjust-sourcemap-loader@^1.1.0:
     object-path "^0.9.2"
     regex-parser "^2.2.1"
 
-aetna@^1.0.0-alpha.17:
+aetna@^1.0.0-alpha.20:
-  version "1.0.0-alpha.17"
+  version "1.0.0-alpha.20"
-  resolved "https://registry.yarnpkg.com/aetna/-/aetna-1.0.0-alpha.17.tgz#e2a97f1dbd0fa47f9e2d9a68f77b662addf3cf6c"
+  resolved "https://registry.yarnpkg.com/aetna/-/aetna-1.0.0-alpha.20.tgz#c9cd27eb15388abe768c6f39b900610050c77e75"
   dependencies:
     "@frctl/fractal" "^1.1.7"