Release 1.2

Here's a minor release.

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### Bug Fixes

* Use /bin/sh for the autogen.sh script, to improve cross-platform
  compatibility. (Sam)

### Docs Improvements

* Added installation instructions for Arch Linux (Antony) and NixOS. (Sam)
* Removed an obsolete (and confusing) notice about release tarballs. (Mathieu)

### Testing improvements

* Get Travis CI to build against the 1.x branch of libcangjie, as libcangjie
  master has now broken tha API in a way that pycangjie 1.x doesn't build
  against it any more. (Mathieu)
* Improve testing coverage. (Mathieu)