Pass remote URL with OAuth token when pushing a branch in Sandbox

This commit should allow branches to be pushed to the sandbox fork without
setting SSH keys in the Travis script.
Separate rebase test with or without push to Github
Filter out tests writing back to Github (using Push regexp) from Travis
Split further the rebase tests (--no-push and --no-pr options)
Exclude all rebase tests from Travis build

Rebase tests explicitely require a new PR to be opened which is incompatible
with the read-only token used for testing.
merge master -D org -I include -E exclude: PR 89 (Tag release)