Bump rails deps requirements to 5.0+

[fgrehm]

Follow up of #92

Before someone attempts to use the gem on Rails < 5 lets update the runtime deps on our gemspec

letter_opener_web/letter_opener_web.gemspec

Lines 23 to 25 in 968d1a0

	gem.add_dependency 'actionmailer', '>= 3.2'
	gem.add_dependency 'letter_opener', '~> 1.0'
	gem.add_dependency 'railties', '>= 3.2'

[fgrehm]

See #113