Fixed an error with Jekyll 2.3.0 (and probably with prior versions) that crashes when building the site

I was getting this error:

```
jekyll 2.3.0 | Error:  undefined local variable or method `path' for #<Jekyll::Layout:0x007fda29134420>
```
Fixed by correctly redeclaring the Layout class inheriting by the original one.

@yaegashi and @felixkiss solutions are integrated as well.