Removes tmp_dir at the end of Genius.save_artists

[hhkarimi]

After saving the lyrics in JSON files inside to the users selected directory, the tmp_dir (./tmp_lyrics), is removed.

Note: this is my first PR so please let me know if there's anything missing!

[johnwmillr]

Thanks for your contribution, Hussain!

I didn't realize I was leaving the tmp_lyrics directory sitting around after calling save_artists. Your code looks good, I'll merge the changes now.

[hhkarimi]

Happy to contribute!

I also noticed that the build on Travis kept failing since the access token wasn't available. I looked into it a bit; it look like you can include encrypted variables in the .travis.yml file using their cli tool: https://docs.travis-ci.com/user/encryption-keys/

Not sure if that makes sense here, but hope it helps!