Update dependency / Fix README

* Update [tempfile](https://github.com/sindresorhus/tempfile) to `^1.0.0`
* Update README to fix incorrect usage of [.dest()](https://github.com/kevva/exec-buffer/tree/82db429e199a2025bb40defa2b2c7843d47a54ce#destpath) and [.src()](https://github.com/kevva/exec-buffer/tree/82db429e199a2025bb40defa2b2c7843d47a54ce#srcpath)