Fix printf format

[BanzaiMan]

"%b" allows the argument to include escaped sequences such as "\n"

[BanzaiMan]

Supersedes #47.

[native-api]

Then you need to escape backslash sequences each time you include external text into a message. Otherwise, any such sequences that happen to be in that text will be expanded, too, which is undesired.

[native-api]

https://github.com/travis-ci/casher/pull/50