[travis] gcc now uses 2 procs (instead of 3).

Recently, gcc has been failing with 3 processes.
[travis] Build simbody with 3 procs.
[travis] Unconditionally build simbody with clang

Even if building opensim with gcc, build simbody with clang. To reduce build time.

Also, remove the SIMBODY_STANDARD_11 cmake variable, which is now gone from simbody.
[travis] simbody uses 8 procs b/c using clang.
Merge pull request #298 from opensim-org/travis-gcc-2-procs

[travis] Simbody compiles with Clang, remove SIMBODY_STANDARD_11 flag.