Merge branch 'master' into ci

* master: (120 commits)
  Fix vignette error
  Bump version number
  add back libgsl-dev, if zinbwave is to be installed
  also install suggested packages on travis in order to be able to run all tests and examples
  moving zinbwave to suggests and removing  apt installation of gsl
  Fix zinbwave installation error
  Bioconductor 3.6 release
  add back all checks
  remove comma
  temporarily reduce build time
  added apt-get dependency for zinbwave
  Add zimbwave to imports
  bump x.y.z versions to odd y after creation of RELEASE_3_6 branch
  bump x.y.z versions to even y prior to creation of RELEASE_3_6 branch
  Version 1.1.8
  Update lun2 reference
  Update citation
  Add BASiCS to lists
  Run checks
  Convert to SingleCellExperiment
  ...