Update gemspec

Remove post_install_message

Set required_ruby_version 2.3

Author: kojix2

iruby.gemspec

@@ -1,14 +1,11 @@
-# coding: utf-8
 require_relative 'lib/iruby/version'
-require 'date'
 
 Gem::Specification.new do |s|
   s.name          = 'iruby'
-  s.date          = Date.today.to_s
   s.version       = IRuby::VERSION
   s.authors       = ['Daniel Mendler', 'The SciRuby developers']
   s.email         = ['mail@daniel-mendler.de']
-  s.summary       = 'Ruby Kernel for Jupyter/IPython'
+  s.summary       = 'Ruby Kernel for Jupyter'
   s.description   = 'A Ruby kernel for Jupyter environment. Try it at try.jupyter.org.'
   s.homepage      = 'https://github.com/SciRuby/iruby'
   s.license       = 'MIT'
@@ -18,19 +15,15 @@ Gem::Specification.new do |s|
   s.test_files    = s.files.grep(%r{^test/})
   s.require_paths = %w(lib)
 
-  m = "Consider installing the optional dependencies to get additional functionality:\n"
-  File.read('Gemfile').scan(/gem\s+'(.*?)'/) { m << "  * #{$1}\n" }
-  s.post_install_message = m << "\n"
+  s.required_ruby_version = '>= 2.3.0'
 
-  s.required_ruby_version = '>= 2.1.0'
+  s.add_dependency 'bond', '~> 0.5'
+  s.add_dependency 'multi_json', '~> 1.11'
+  s.add_dependency 'mimemagic', '~> 0.3'
+  s.add_dependency 'data_uri', '~> 0.1'
 
   s.add_development_dependency 'rake'
   s.add_development_dependency 'minitest'
   s.add_development_dependency 'ffi-rzmq'
   s.add_development_dependency 'pycall', '>= 1.2.1'
-
-  s.add_runtime_dependency 'bond', '~> 0.5'
-  s.add_runtime_dependency 'multi_json', '~> 1.11'
-  s.add_runtime_dependency 'mimemagic', '~> 0.3'
-  s.add_runtime_dependency 'data_uri', '~> 0.1'
 end