feat(lein): upgraded the embedded Leiningen version: 2.1.2 -> patched 2.3.4

Leiningen 2.3.4 applied patches:
- nREPL URL appended to default message notifying user of nREPL host/port
feat(lein classpath): Resolving leiningen classpath is now more permissive

- Overlapping :source-paths | :resource-paths | :test-paths
              | :java-source-paths is accepted, and resolved by adding the
              required exclusions (e.g. if you declare both "resources"
              and "resources/public" as resource paths, Eclipse will create
              2 source path entries: one for "resources/public", and one
              for "resources - excluding resources/public"
- All paths are marked as "optional" so that if folders for these paths
  are missing, no exception is reported by Eclipse.