ruby: use ruby's built-in support to test for infinity and nan
Merge pull request #3 from rock-core/ruby_remove_custom_inf_and_nan

ruby: use ruby's built-in support to test for infinity and nan
test: depend on minitest only

There's a collision between ruby's built-in minitest library and the
gem (they are incompatible), which breaks testrb and testing in general.

Switch to using only minitest