Support closing remote subprocess' stdin explicitly

[bitprophet]

This is covered by pyinvoke/invoke#552 and requires it (specifically, its support for adding a Runner.close_proc_stdin method - so this happens to be backwards compatible insofar as it won't error on older Invoke versions).

Just making this so I've got something for the changelog really.

[bitprophet]

TODO: run tests on travis after Invoke 1.3.0 is released.

[bitprophet]

As seen in #1989 I just sucked it up for now and added master-invoke to our dev-requirements.txt, so that Travis will test against that branch instead of latest-public-release.

Tests seem to pass, so this is done for reals now I think.