Fix package check: checking for hidden files and directories ...

```
...
✔  checking for executable files (529ms)
N  checking for hidden files and directories ...
   Found the following hidden files and directories:
     .travis.yml
     .github
   These were most likely included in error. See section ‘Package
   structure’ in the ‘Writing R Extensions’ manual.
✔  checking for portable file names ...
...
```