project root dir is ignored when used with projectile

[mbuczko]

This can be painful when clojure-mode is used together with projectile and cider. For certain operations (like jack-in to the REPL) cider tries to find a project root dir via clojure-project-dir and in most scenarios it works. The problem however appears when project uses few other build tools at the same time.

Imagine following directory structure:

/proj
|-> build.boot
|-> subproject_1
      |-> build.gradle

Projectile allows to customize a way how the project root dir is calculated and having build.gradle files ignored in this case it locates /proj root with build.boot inside correctly. Unfortunately, as clojure-project-dir doesn't use that "knowledge" it simply forces subproject_1 as a root dir (when called within this directory) and jack-in goes with gradle.

I prepared a simple PR which mitigates this problem. Please, take a look at it and accept if it makes sense.