feat(aXe): update aXe version to 2.6.1

Also, use a [tilde range](https://docs.npmjs.com/misc/semver#tilde-ranges-123-12-1)
to avoid being impacted by rule behavior changes.

Fixes #148

packages/ace-core/package.json

@@ -21,7 +21,7 @@
     "@daisy/ace-report-axe": "^0.8.0",
     "@daisy/epub-utils": "^0.8.0",
     "@daisy/puppeteer-utils": "^0.7.0",
-    "axe-core": "^2.5.0",
+    "axe-core": "~2.6.1",
     "h5o": "^0.11.3",
     "p-map": "^1.2.0",
     "puppeteer": "^0.12.0",

yarn.lock

@@ -287,9 +287,9 @@ aws4@^1.2.1, aws4@^1.6.0:
   version "1.6.0"
   resolved "https://registry.yarnpkg.com/aws4/-/aws4-1.6.0.tgz#83ef5ca860b2b32e4a0deedee8c771b9db57471e"
 
-axe-core@^2.5.0:
-  version "2.5.0"
-  resolved "https://registry.yarnpkg.com/axe-core/-/axe-core-2.5.0.tgz#2f7c662c4d7d07cc8dc81f2f1d3d8288d8628039"
+axe-core@~2.6.1:
+  version "2.6.1"
+  resolved "https://registry.yarnpkg.com/axe-core/-/axe-core-2.6.1.tgz#28772c4f76966d373acda35b9a409299dc00d1b5"
 
 babel-code-frame@^6.16.0, babel-code-frame@^6.26.0:
   version "6.26.0"