* Add python3-sys to rust-cpython as an optional feature, and
make python27-sys also optional, but still the default
* Parametrise python27-sys/build.rs so that it is python
version independent, and clone it into python3-sys/build.rs.
Hopefully this can continue to be maintained as an identical
file.
* python27-sys and python3-sys gain features for explicitly
selecting a python version to link to. for python27-sys,
there's currently only python27; for python3-sys there's
python 3.4 and 3.5.
* explicitly tell travis to use nightlies (seems to have
started trying to use 1.0.0)
