Bump to next stable Nextflow version when available

[drpatelh]

In order to fix issues observed whilst converting Docker -> Singularity images in v2.0 I have now implemented the direct download of Singularity images in #531 which seems to be working well as reported by a number of users. However, I needed to use one of the latest edge versions of Nextflow to be able to use this functionality (see #496). When the next, stable version of Nextflow is released we need to bump the version used in this pipeline. I am sure this will happen anyway but just putting this here as a reminder.