TimeStamp.new now takes {time: time_string} option

* moved the from_s method to a private method
  replaced by TimeStamp.new(time: time_string)
Bugfix: Fact.time_stamp was a string when initialized

* now using the formal setter (with the validation)
* since TimeStamp.new now takes a string (previous commit),
  we use that function to build Facts from time_string
* added Fact#string_values
* replaced Fact.from_values => Fact.from_string_values
Unrolling the setters to avoid a 12% performance penalty

* previous commit 5e279a3 introduced 3 setters with set_once,
  this caused an increase from 47 to 54 us per fact.
* the set_once is not really needed for the initializer
Fact#equivalent?

* 5 attributes exactly the same
* the time_stamp is max  within 1000 ns from other fact
  (to allow slight drift when merging fact streams)
Factories::Resource.empty
Release most versions in th dbd.gemspec

* to try to fix this on Travis:
  $ gem query --local | grep bundler >/dev/null || gem install bundler

  ERROR: Could not find a valid gem 'bundler' (>= 0), here is why:

  Unable to download data from https://rubygems.org/ - no such name (https://rubygems.org/latest_specs.4.8.gz)