Rename repository to 'qiskit-core'

[diego-plan9]

Description

Rename all references to qiskit-sdk-py to the new repository name,
QISKit/qiskit-core, including a handful of instances where the
IBM/qiskit-sdk-py form was used.

Fixes #467

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[diego-plan9]

Please note that after renaming the repository, the old URL will still be working (both in the github interface and in the remotes) - it might be advisable to announce the change a bit, for a cleaner transition and avoid confusion.

The PR also changes the string in some travis related files (the actual .travis.yml and the badge in the README) - it seems travis will be able to pick the renaming, but we will check after it is completed.

[atilag]

PLEASE NOTE: Even though local repository clones will still work after the renaming, it's highly encouraged to update local clones with the new URL:

git remote set-url origin https://github.com/QISKit/qiskit-core

[jaygambetta]

i am sure we missed some but this looks good. Good work @diego-plan9