[readme] Update cmake and simbody dependencies.

Reflects the fact that in Simbody 3.6 no longer has a SIMBODY_STANDARD_11 cmake variable. Also, updated the cmake dependency version to 2.8.6.
Merge pull request #301 from opensim-org/readme-simbody-standard-11

[readme] Update cmake and simbody dependencies.
[ci skip]