Gemfile: delete gems that were needed for Guard

`rb-inotify` has a dependency on `ffi`, which fails to compile on Ruby 1.9.3
while running on CircleCI. I don't think anyone who develops Pry (seems to be
just me so far) uses Guard in their worfklow. Our repo is missing an obligatory
Guardfile, too, which makes me think it's a very safe change to make (rather
than wasting time with`ffi` and CircleCI).

Gemfile

@@ -2,12 +2,9 @@ source 'https://rubygems.org'
 gemspec
 gem 'rake', '~> 10.0'
 
-# For Guard
 group :development do
   gem 'gist'
   gem 'yard'
-  gem 'rb-inotify', require: false
-  gem 'rb-fsevent', require: false
 
   # Rubocop supports only >=2.2.0
   if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.2.0')