resolves #308 set proper encoding on input data

- set constant that determines whether to force encoding
- force encoding on all input data
- test to verify input data is properly encoded
- new rake task to run tests w/ US-ASCII default external encoding
check Encoding.default_external, not IO#external_encoding
fix warning
use explicit ruby in invoker test
small optimization
update travis config
Merge pull request #310 from mojavelinux/force-encoding

resolves #308 set proper encoding on input data