Peg anchor-markdown-header to ^0.5.5 (#110)

Fixes #81.
Fixes #104.

This version includes changes that correctly handle '@' signs in
headers.

package.json

@@ -16,7 +16,7 @@
     "node": ">=0.4"
   },
   "dependencies": {
-    "anchor-markdown-header": "^0.5.4",
+    "anchor-markdown-header": "^0.5.5",
     "htmlparser2": "~3.7.1",
     "markdown-to-ast": "~3.2.3",
     "minimist": "~1.1.0",