Fix conversions to work with Go 1.15

[batterystaples]

As described in https://github.com/golang/go/issues/32479, as of Go 1.15, a warning will be raised when a program attempts to use a string(int) type conversion. For this project, this causes "make test" to fail. This pull request uses strconv methods to convert the values instead, which causes the warning to no longer appear. This pull request fixes #218.