Add rake timescaledb:update_extension task.

[jonatas]

It relates to #111 and #112.

[exterm]

two thoughts:

• I don't have a DATABASE_URL env var. Rails apps usually keep database configuration in config/database.yml, but you should be able to read it from Rails.configuration at run time
• instead of manually requiring the rake task from the Rakefile, have you considered using a railtie to inject it from the gem? The railtie could also inject an initializer to initialize the TimescaleDB connection.