Add support for runtimes versioning

Description

At the moment we do not version runtimes. One can specify following runtime:

functions:
  my-func:
    handle: Something.run!
    runtime: https://gitlab.com/gitlab-org/serverless/runtimes/ruby

gitlabktl then clones Git repository, and writes file into the memory (issue about improving that ➡ #23 (closed)). Then gitlabktl is going to check master branch.

Proposal

We should add support for versioning:

Make it possible to specify a version of a runtime.

When using a tag in Git repository:

functions:
  my-func:
    handle: Something.run!
    runtime: gitlab/serverless/runtimes/ruby:0.1

When using a branch in Git repository:

functions:
  my-func:
    handle: Something.run!
    runtime: gitlab/serverless/runtimes/ruby@development

/cc @DylanGriffith @danielgruesso