updating version numbers for 4.9.0 rc

[serban-nicusor-toptal]

Summary

Update version numbers for 4.9.0 release. Release notes to be added later.

Tests

Side Effects

Release notes

Checklist

• [ ] Copyright holder: (fill in copyright holder information)

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:

  • Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  • Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• [ ] the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• [ ] the code is written in idiomatic C++ and changes are documented in the doxygen

• [ ] the new changes are tested