Use Leiningen's task dispatch instead of starting the REPL directly

These changes simplify lein-try's code structure and internal logic a little bit. Changes include:

- add new dependencies to the profile `:try` instead of the global dependencies
- start REPL using `leiningen.core.main/apply-task`
- declare lein-try as `^:higher-order`

On my machine (...), both Leiningen 2.1.3 and 2.2.0 let the plugin operate correctly. 

Also, issue #11 is addressed, making it possible to use custom data readers _within_ existing projects. It seems as if some compilation/resolution step is missing when calling lein-try in a non-project environment. This is indicated by different startup times - taking longer within a project than outside - and maybe the output produced by `DEBUG=1 lein try ...` which includes the two (project-only) tasks `javac` and `compile`.