r-arrow compatibility issues.

[BoPeng]

I cannot yet reproduce here but our test suite fails on both travis-CI and AppVeyor.

On Travis, I saw

 ValueError: Cell produces error message: Failed to get variable: module 'pyarrow' has no attribute 'compat'. Use expect_error=True to suppress this error if needed.

which is a sign that an old version of pyarrow is installed (see this).

On AppVeyor, another set of tests fail.

I would suggest that we test sos-r on more systems before we make a new release.

[BoPeng]

Windows tests have been fixed.

The Linux one (travis) needs more work.