Syntax error in SourceHgWeb.php with PHP 5.5

```
Parse error: syntax error, unexpected '.', expecting ',' or ';' in .../plugins/source-integration/SourceHgWeb/SourceHgWeb.php on line 260 
```

Caused by assignment in static variable declaration (only supported on >= PHP 5.6 as per [documentation](http://php.net/manual/en/language.variables.scope.php#language.variables.scope.static))