Use released version of rustls dependency

[meqif]

Currently master fails to build with:

Updating git repository `https://github.com/ctz/ring`
error: failed to load source for a dependency on `ring`
Caused by:
  Unable to update https://github.com/ctz/ring#9a468a7a
Caused by:
  failed to clone into: /buildslave/target/cargo-home/git/db/ring-d33dffdb083ffa66
Caused by:
  failed to authenticate when downloading repository
attempted to find username/password via git's `credential.helper` support, but failed
To learn more, run the command again with --verbose.

I tracked ring down as a dependency of rustls in download. Updating rustls to the latest version (0.1.2) fixes the issue.

I'm not sure how unstable rustls's API is, so I took the safe bet of clamping down to the exact version instead of relaxing to 0.1. Feel free to tell me to change it, though!

[meqif]

Should I check in the changes to Cargo.lock as well?

[meqif]

Before checking in the lockfile changes, the Windows builds succeeded but the Travis builds failed with:

error: failed to write /buildslave/Cargo.lock
Caused by:
  failed to open: /buildslave/Cargo.lock
To learn more, run the command again with --verbose.

After adding them it seems like those build issues are solved (at the time of writing this comment, the tests are running and at least one succeeded in Travis before squashing).

I squashed the individual commits. Please let me know if you'd like me to change anything.

[brson]

Thanks @meqif!