bump to version '0.3.1'

Signed-off-by: Sebastien Varrette <sebastien.varrette@uni.lu>
Update Gemfile.lock accordingly

Signed-off-by: Sebastien Varrette <sebastien.varrette@uni.lu>
Merge branch 'release/0.3.1' into devel

* release/0.3.1:
  Update Gemfile.lock accordingly
  bump to version '0.3.1'