Fix travis CI

[arvoelke]

The first commit makes our pytest.raises checks more strict. This is something I was using to debug, and isn't strictly needed by this PR, but IMO is a good thing to do.

The second commit localizes the os.chdir to be within a try-finally block. This avoids a really weird issue we were seeing as a part of #217 (e.g., see this job and look for nengo_loihi/hardware/tests/test_allocators.py::test_block_size).

Essentially... if an error happens when the model is being built, then the current working directory (cwd) is permanently changed. More specifically, an error occurs within HardwareInterface.__init__, and so __exit__ is never invoked. As a result, the directory is never changed back. This only occurs for --target loihi, and causes coverage.xml to be written to the wrong directory. Part of what made this issue weird is that coverage.xml is only written to the wrong directory if -n is omitted, but not for -n 1. This may be a subtle aspect of how pytest spins up its workers.

The second commit also adds a check to pytest's setup/teardown to make sure the directory is not changing. This test would fail on test_block_size with --target loihi if it were not for the fix.

Note this does not solve #217 (i.e., it does not make nengo-loihi process safe). The issue explained in the commit message of e9cd851 is still present. Solving this is outside the scope of PR.

[hunse]

I think we should be doing the chdir in create_io_snip, which gets called when run is called, not when the HardwareInterface is created. It has the added advantage of keeping all the snip stuff in one place. I don't think there's any need to change the directory in __init__.

[arvoelke]

Wait why do we even chdir at all? It must have been necessary at some point, but that no longer seems to be the case. Everything passes with nxsdk-0.8.0 without the chdir. I added a couple commits to demonstrate.

[hunse]

Yeah, I thought I remembered Intel trying to fix that, though I don't ever remember them conclusively saying they did. But if it works without it for 0.8, then let's just get rid of it. We can probably drop support for pre-0.8 by this point. It was definitely necessary with some earlier version.

[drasmuss]

Added a commit to drop support for nxsdk<0.8.0, as well as updating the installation instructions and testing against 0.8.1. With that, this LGTM!