minor: whitespace
avoid using octal constants

Using "01" is unwise, since using a leading zero indicates an octal
(base 8) number.
Removed some 'assigned but unused' variables.
Merge branch 'avoid-octal' of https://github.com/xpe/finance into xpe-avoid-octal

Conflicts:
	test/test_cashflows.rb

Removed octal constants while keeping minitest structure.