Add `context` method as `describe` alias

This PR adds `context` method as alias of `describe` that is like RSpec style.

[I already added PR like this one to `alloy/MacBacon`](https://github.com/alloy/MacBacon/pull/2) and it was merged to master branch.
If spec.rb in RubyMotion will be updated by MacBacon's master branch code, you can ignore this request.
(I don't know a relation between spec.rb in RubyMotion and MacBacon, sorry. :sweat: )