fix: Gemfile & Gemfile.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-ACTIVESUPPORT-569598

Gemfile

@@ -1,13 +1,13 @@
 source 'https://rubygems.org'
 
-gem 'activerecord'
+gem 'activerecord', '>= 5.2.4.3'
 gem 'sinatra'
 gem 'sqlite3'
 
-gem 'sinatra-activerecord'
+gem 'sinatra-activerecord', '>= 2.0.13'
 gem 'sinatra-contrib', require: false
 gem 'sinatra-flash'
-gem 'validate_url'
+gem 'validate_url', '>= 1.0.2'
 
 gem 'slim'
 
@@ -21,7 +21,7 @@ gem 'tzinfo-data'
 group :development do
   gem 'rspec'
   gem 'poltergeist'
-  gem 'factory_girl'
+  gem 'factory_girl', '>= 4.8.0'
   gem 'database_cleaner'
   gem 'launchy'
   gem 'pry'