Require at least Mojolicious 4.50 due to changes in Mojo::UserAgent

Hello,

Mojolicious::Command::deploy::heroku wasn't working in the current version of Mojolicious due to a change in the Mojolicious::UserAgent module

According to the CHANGES file in Mojolicious:

```
4.50  2013-10-22
  - Deprecated Mojo::UserAgent::app in favor of Mojo::UserAgent::Server::app.
```
This pull request increments the Mojolicious version in the "dist.ini" to 4.50 file and changes the code in "heroku.pm" to use the correct Mojo::UserAgent::Server methods.