Slight memory reduction

[AaronLasseigne]

closes #54

Doing a basic get showed a slight reduction in objects created.

Using:

require 'rubygems'
require 'memory_profiler'
require 'tzinfo'

MemoryProfiler.report(ignore_files: 'rubygems') {
  TZInfo::Timezone.get('America/New_York')
}.pretty_print

Before:

allocated memory by gem
-----------------------------------
   1294730  tzinfo
        40  concurrent-ruby-1.0.2
        40  other

After:

allocated memory by gem
-----------------------------------
   1225130  tzinfo
        40  concurrent-ruby-1.0.2
        40  other

5.38% lower memory usage

[philr]

Thank you for submitting this pull request. I've merged it into master.