Revised pull request section in `CONTRIBUTING.md`

Adding mention of Travis and AppVeyor per Daniel Lopes (@danielsimoeslopes) in #271.
I also make the main PR activities bold.

Daniel, please take a look [here](https://github.com/simbody/simbody/blob/revise-pr-section-in-contributing/CONTRIBUTING.md#mechanics-of-submitting-a-pr) and let me know if this is what you had in mind.

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