Dropbox download url parameter fix for unwanted beta builds

[AllPurposeBen]

Ref this issue: https://github.com/autopkg/recipes/issues/473

• Changes from using a download URL that is hardcoded in the process section to using input variables that can be overridden as desired
  • Allows overriding the both the build type (stable vs beta, possibly others?) parameter
  • Allows overriding the download URL value wholesale
• Adds a parameter to the download URL for the build type explicitly, which seems to always provide the requested build vs the current behavior that randomly gives you a beta build instead of a stable.

[nmcspadden]

Thanks for looking into this!