Add package 'zenith' to Suggests

[HenrikBengtsson]

The 'variancePartition' package lacks 'zenith' under Suggests:, because the vignette fails to build if package 'zenith' is not installed.

* checking re-building of vignette outputs ... ERROR
Error(s) in re-building vignettes:
--- re-building ‘FAQ.Rmd’ using rmarkdown
--- finished re-building ‘FAQ.Rmd’

--- re-building ‘additional_visualization.Rmd’ using rmarkdown
--- finished re-building ‘additional_visualization.Rmd’

--- re-building ‘dream.Rmd’ using rmarkdown
--- finished re-building ‘dream.Rmd’

--- re-building ‘errors.Rmd’ using rmarkdown
--- finished re-building ‘errors.Rmd’

--- re-building ‘mvtests.Rmd’ using rmarkdown

Quitting from lines 118-127 [zenith] (mvtests.Rmd)
Error: processing vignette 'mvtests.Rmd' failed with diagnostics:
there is no package called 'zenith'
--- failed re-building ‘mvtests.Rmd’

--- re-building ‘rnd_effects.Rmd’ using rmarkdown
--- finished re-building ‘rnd_effects.Rmd’

--- re-building ‘variancePartition.Rmd’ using rmarkdown
--- finished re-building ‘variancePartition.Rmd’

SUMMARY: processing the following file failed:
  ‘mvtests.Rmd’

Error: Vignette re-building failed.
Execution halted