pip release: 0.6.1 (Qiskit#1037)

- There are no changes in the code. Just a rebuild of the wheel package.

Merge pull request Qiskit#1025 from atilag/stable-0.6

Stable 0.6

pip release: 0.5.7

Merge pull request Qiskit#628 from diego-plan9/stable-0.5.6

Stable 0.5.6

Merge pull request Qiskit#617 from atilag/stable-0.5.5

Stable 0.5.5

Merge pull request Qiskit#559 from atilag/stable-0.5.4

Stable 0.5.4

Merge pull request Qiskit#517 from atilag/stable-0.5.3

Stable 0.5.3

Merge pull request Qiskit#488 from atilag/stable-0.5.2

Stable 0.5.2

Merge pull request Qiskit#470 from atilag/stable-0.5.1

pip release: 0.5.1

pip release: 0.5.0