fix increasing indent on error tracebacks

[pushcx]

@joeldrapper and I discussed the increasing error indent I mentioned in #59 and he said it wasn't a desired style. Here's the update:

./test/invocation.test.rb:37 call
  unexpected ArgumentError, wrong number of arguments (given 1, expected 0)
  ./test/invocation.test.rb:37:in 'block (3 levels) in load_tests'
  /home/pushcx/code/hotcake/lib/hotcake.rb:86:in 'Invocation#call'

Passed: 32 | Failed: 0 | Errors: 1