Update dependencies

[eight04]

Fixes #9.

Update dependency to the latest. npm audit still warns with outdated bl module:

=== npm audit security report ===                        
                                                                                
                                                                                
                                 Manual Review                                  
             Some vulnerabilities require your attention to resolve             
                                                                                
          Visit https://go.npm.me/audit-guide for additional guidance           
                                                                                
                                                                                
  Moderate        Memory Exposure                                               
                                                                                
  Package         bl                                                            
                                                                                
  Patched in      >=0.9.5 <1.0.0 || >=1.0.1                                     
                                                                                
  Dependency of   level-sublevel                                                
                                                                                
  Path            level-sublevel > levelup > bl                                 
                                                                                
  More info       https://nodesecurity.io/advisories/596                        
                                                                                
found 1 moderate severity vulnerability in 1095 scanned packages
  1 vulnerability requires manual review. See the full report for details.

level-filesystem@1.2.0 D:\Dev\level-filesystem
`-- level-sublevel@6.6.2
  `-- levelup@0.19.1
    `-- bl@0.8.2 

Ref: dominictarr/level-sublevel#114

[eight04]

A PR to remove levelup dependency:
dominictarr/level-sublevel#116