Add sqlite3 to Gemfile

Fixes all the post modules that require it to parse pilfered sqlite DB
files.

VERIFICATION
 - [x] `bundle install`
 - [x] launch msfconsole
 - [x] get a windows meterpreter session, note its session ID
 - [x] `use post/multi/gather/skype_enum`
 - [x] `set SESSION ` to the appropriate ID from above
 - [x] `run`
 - [x] **VERIFY**: no errors about missing sqlite3