Data mentions facebookposts and facebookcomments

### Added
- data/mentions/facebookcomments endpoint
- data/mentions/facebookposts endpoint

### Updated
- removed redundant else as per rubocop error
`lib/bwapi/error.rb:28:7: C: Redundant empty else-clause.`

Tested this locally
```
[61] pry(main)> if false
[61] pry(main)*   true
[61] pry(main)* elsif false  
[61] pry(main)*   true
[61] pry(main)* end  
=> nil
```

Ran rubocop and rspec as per guidelines
Also ran against current work to test methods