Fix Date & DateTime parsings to use default timezone

Date.new(...) does not take into account the current timezone, while other
parts of the application do. By default always parsing any date with the
default timezone and converting the resulting Time to Date would prevent
this kind of issues

DateTime.parse(...).in_time_zone gives an unexpected result, as the
DateTime.parse(...) will create a DateTime with +0000 time zone and the
`in_time_zone` will modify the DateTime to adjust to the default zone.

Maybe its better explained with an example, using 'Lima' as timezone:

DateTime.parse("2015-01-01")
> Thu, 01 Jan 2015 00:00:00 +0000

DateTime.parse("2015-01-01").in_time_zone
> Wed, 31 Dec 2014 19:00:00 -05 -05:00

And that's not the desired date but the previous day!

spec/models/ahoy/data_source_spec.rb

@@ -3,9 +3,9 @@ require 'rails_helper'
 describe Ahoy::DataSource do
   describe '#build' do
     before do
-      time_1 = DateTime.parse("2015-01-01").in_time_zone
-      time_2 = DateTime.parse("2015-01-02").in_time_zone
-      time_3 = DateTime.parse("2015-01-03").in_time_zone
+      time_1 = Time.zone.local(2015, 01, 01)
+      time_2 = Time.zone.local(2015, 01, 02)
+      time_3 = Time.zone.local(2015, 01, 03)
 
       create :ahoy_event, name: 'foo', time: time_1
       create :ahoy_event, name: 'foo', time: time_1