Remove date field from rake.gemspec

Hi there 👋

I noticed that in [this commit](6258ad5) you bumped the version but did not change the `date` field in the `rake.gemspec`. This means that on rubygems.org, Rake 12.2.1 and Rake 12.3.0 were seemingly released on the same day, but the commits show a different history. (The last released date was [this commit](e7ea2d1) but it has since been bumped on master with [this commit](0c4aab8).


May I suggest removing the `date` field from the `gemspec`? This way, RubyGems.org will automatically assign the current date to the package's release _and_ it means that you as a maintainer will have to do one less thing every time you release a new version. We do not have a `date` field in the [`i18n` gem's `gemspec`](https://github.com/svenfuchs/i18n/blob/master/i18n.gemspec) and it works just fine. 

What do you think?

rake.gemspec

@@ -5,7 +5,6 @@ require 'rake/version'
 Gem::Specification.new do |s|
  s.name = "rake".freeze
  s.version = Rake::VERSION
- s.date = "2017-11-15"
  s.authors = ["Hiroshi SHIBATA".freeze, "Eric Hodel".freeze, "Jim Weirich".freeze]
  s.email = ["[email protected]".freeze, "[email protected]".freeze, "".freeze]