Fix clash with dotenv gem that prevents applications from starting.

[Dotenv](https://github.com/bkeepers/dotenv) is a gem that helps Rails applications keep their configuration in environment variables, and loads them automatically upon application startup. This configuration is stored in a file called `.env`. Prax also tries to load environment variables from `.env` upon startup. 

If your environment variables are both in a Bash-friendly format, this doesn't matter. But Dotenv allows users to use YAML to store their config; Bash can't source this, which causes the app to fail to start without outputting anything into the log. 

This pull request checks for the existence of Dotenv in your bundle. If it finds it, then Prax will not load your `.env` file. Instead, it leaves that up to Dotenv. 

This should fix https://github.com/ysbaddaden/prax/issues/95.