Avoid RegExp when we can use simple JS

lib/previous-map.js

@@ -51,7 +51,8 @@ class PreviousMap {
       return fromBase64(text.substr(baseUriMatch[0].length))
     }
 
-    let encoding = text.match(/data:application\/json;([^,]+),/)[1]
+    let encoding = text.slice('data:application/json;'.length)
+    encoding = encoding.slice(0, encoding.indexOf(','))
     throw new Error('Unsupported source map encoding ' + encoding)
   }