Allow specifying sqlite version

Older versions of activerecord require sqlite ~> 1.3.6, but not in the
gemfile, the constraint is enforced when you load the adapter.

This allows us toe specify in the environment the version of the gem
we'd like to run against.

Gemfile

 source 'https://rubygems.org'
 
-gem 'sqlite3', platforms: [:ruby]
+sqlite = ENV['SQLITE_VERSION']
+
+if sqlite
+  gem 'sqlite3', sqlite, platforms: [:ruby]
+else
+  gem 'sqlite3', platforms: [:ruby]
+end
 
 platforms :jruby do
   gem 'activerecord-jdbcsqlite3-adapter'