Issue 78 fix meta for latest pip

[delapuente]

Fix #78

Forces the process to finish with an error (which is semantically more accurate) making pip fallback to installing from sources.

[mtreinish]

LGTM, I tested this locally and the qiskit package is installable (albeit with an ugly error displayed by pip) with the latest version of pip, 19.0.2. Without this patch it fails as reported in #78.

I do hope at some point we can remove this workaround to reinstall terra, because it makes debugging install issues much trickier.

[derivation]

@mtreinish do you think it worth while to add an Issue calling for a more direct solution in your own words?

[mtreinish]

@derivation I'm not sure there is an alternative, the need for the workaround is caused by the qiskit python package owning the qiskit python namespace before the 0.7, which isn't a condition that will ever change. If we want to make sure there is a seamless upgrade process from anyone using qiskit<0.7 to a newer version we'll probably always need that. My comment before was just my idle thoughts on how I dislike that it's needed at all.

[derivation]

@mtreinish now I understand :)