Remove inaccurate note.

`yield_control` supports modifiers like `once`,
`twice`, `exactly(10).times`, etc.
Make `yield_control` default to `at_least(:once)`.

Previously, it raised a confusing error when multiple
yields occurred:

> The yield_control matcher is not designed to be used with a
> method that yields multiple times. Use the yield_successive_args
> matcher for that case.

This was confusing because yield_control has supported multiple
yields for awhile.  This also simplifies the implementation!
Merge pull request #675 from rspec/improve-yield-control-matcher

Make `yield_control` default to `at_least(:once)`.
Updated travis build scripts (from rspec-dev)
Merge pull request #680 from rspec/update-travis-build-scripts-2014-11-12-for-master

Updates from rspec-dev (2014-11-12)
Updated travis build scripts (from rspec-dev)
Merge pull request #681 from rspec/update-travis-build-scripts-2014-11-12-for-master

Updates from rspec-dev (2014-11-12)
Updated travis build scripts (from rspec-dev)
Merge pull request #682 from rspec/update-travis-build-scripts-2014-11-13-for-master

Updates from rspec-dev (2014-11-13)
Updated travis build scripts (from rspec-dev)
Merge pull request #683 from rspec/update-travis-build-scripts-2014-11-13-for-master

Updates from rspec-dev (2014-11-13)
Updated travis build scripts (from rspec-dev)
Merge pull request #684 from rspec/update-travis-build-scripts-2014-11-15-for-master

Updates from rspec-dev (2014-11-15)