Replace double quotes with single quotes while adding an entry into Gemfile

The purpose of this commit is to make the quotes consistent in Gemfile.
New Gemfile in rails root directory uses single quotes, but if user uses a template(--template) option to add more gems into Gemfile, then it generates rows with double quotes.
An example of inconsistent Gemfile:
```ruby
source 'https://rubygems.org'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.1.1'
# Use postgresql as the database for Active Record
gem 'pg'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 4.0.3'


group :test do
  gem "technoweenie-restful-authentication", lib: "restful-authentication", source: "http://gems.github.com/"
  gem "rails", "3.0", git: "git://github.com/rails/rails"
end
```
```ruby
# template.rb
gem_group :test do
  gem 'technoweenie-restful-authentication', lib: 'restful-authentication', source: 'http://gems.github.com/'
end
```