Attempt to fix doxygen not able to find dot on OSX build environment.
For Travis CI - misc. tweaks. [ci package]

- Perform shallow clone when building site documentation to speed up cloning process.
- Perform unshallow pull when packaging so that 'git describe' gives correct commit count since last tag.
- Only perform packaging when all the previous steps are successful. Reduce chances of binary package with not up-to-date documentation.
- When packaging for iOS platform, generate the documentation quietly (no stdout but still stream error to stderr).
For Travis CI - switch CI build to use OS X build environment.