Change the way we publish new version

[mfoll]

We should keep the master branch fixed between version releases. Then we have a branch for the next version where we make pull requests from other(s) dev branches. Once the new version is ready we merge it to master and delete the "next version" branch. We can then recreate a "next version" branch and so on. This is linked to #68

[mfoll]

We should think about how CircleCI deploys: for the moment it is doing it only for the dev and master branches.

[mfoll]

Current strategy to publish a new version:

• change version number in bin/needlestack.r to be written in the VCF
• change version number in needlestack.nf info
• change version number in nextflow.config for the default docker container
• create a new release along with tag name corresponding to the new version