Fix for @on memoisation

There is an issue with multiple providers and using `on` to determine when a build should be deployed.

Currently `@on` is calculated once and stored, and unfortunately this leads to issues because the first deploy determines the rest of the deploys `on` settings.

This is a quick fix (more so for discussion). I'd probably prefer to set the `@on` variable on each configured deploy instead of just setting it to `nil`, unless there's a preference for a `reset` method.

As this is happening on a private build I could send it over to someone at `travis-ci` for review. Otherwise I'll write some tests in the morning to highlight the issue.