modify DropboxManager.java

[sa-shiro]

addresses #19 -> add support for large DropBox Uploads (>= 150MiB)

this may need revision as of my testings:

• [x] upload files smaller than 150MiB using backup dropbox upload <NAME>
• [x] upload files larger than 150MiB using backup dropbox upload <NAME>
• [ ] command backup create <NAME> immediately fails with an FTP Upload Error
• [ ] automatic scheduled backups hasnt been tested and may result in the same error as above

this shouldnt happen at all? maybe there was something wrong on my end during build, idk