Rename pip-requires to requirements.txt.

Change-Id: I4cfdd0ef6cddfa3d75b4d650362f9645486755c8
Add requirement on pip.

pbr depends on pip, as does much of the OpenStack development
infrastructure. Be explicit about it.

Change-Id: I23119c1856dc05a78940f8ea75cc4d6be07e29f6