Bug invoice generation #392

Bounty: (BUG: Invoice generation fails for paid teams) [https://assembly.com/coderwall/bounties/392]

Problem: mail template rendering was failing due to `undefined method for Nil...`.  
This was happening because of trying to access fields in Stripe response which were not available (due to API change?)

Changes:
- dev env: added `turnip` and `selenium` driver.
- added flash error if invoice generation is unsuccessful
- extracted some code from mail template into `AccountsHelper`
- removed redundant `subscription_mailer/team_upgrade.text.erb` template
- added Fabricators for ':plan`, `:account`, `:team'
- covered with specs and features