Upgrade from outdated tempfile crate

[glebpom]

No description provided.

[stepancheg]

Note travis-ci failures.

[stepancheg]

OK, why do we need to upgrade tempfile to tempdir? I've just opened tempdir crate homepage and haven't found anything wrong: the project seems to be alive and there is no deprecation or suggestions to migrate to tempfile.

[glebpom]

hey, sorry, that's a mistake. Just got messed with different variants of temp files management crates. The goal was to stop using outdated rand crate (v0.4), which was a dependency of tempdir (instead of tempfile, heh), which was used by protoc-rust version 2.2.2. In fact, it has already been fixed in a master version of protoc-rust. Closing the PR.

[stepancheg]

@glebpom I'm not sure what's fixed in master, but if some commit needs to be ported to stable branches, please open an issue or PR, because master won't be released any time soon. Thanks!

[glebpom]

Please, take a look at #374