fix warning Time#succ is obsolete; use time + 1

warning is
1.9.3-p429/lib/ruby/gems/1.9.1/activesupport-3.2.13/lib/active_support/time_with_zone.rb:332:
warning: Time#succ is obsolete; use time + 1

warning thrown in Twitter::Bootstrap::Calendar::BootstrapCalendar#weeks
where a date range is generated by (first..last).to_a

generating dates with ruby's Date#next_date
seems to solve this problem

this issue slows down the rendering also

TODO: check the exact issue