Respect the cider-preferred-build-tool variable

## Expected behavior
With both a build.boot and project.clj,

(setq 'cider-preferred-build-tool 'boot)

M-x cljr-add-project-dependency

The dependency should end up in the build.boot file.

## Actual behavior

This will end up only in the project.clj file

## Steps to reproduce the problem

See "Expected Behavior"

## Environment & Version information

### clj-refactor.el version information

clj-refactor 2.4.0-SNAPSHOT (package: 20170608.320), refactor-nrepl is unreachable

### CIDER version information
;; CIDER 0.15.0snapshot (package: 20170629.106), nREPL 0.2.13
;; Clojure 1.9.0-alpha14, Java 1.8.0_131

### Leiningen or Boot version

Leiningen 2.7.1 on Java 1.8.0_131 Java HotSpot(TM) 64-Bit Server VM
### Emacs version

GNU Emacs 25.2.1 (x86_64-apple-darwin13.4.0, Carbon Version 157 AppKit 1265.21) of 2017-04-29

### Operating system

MacOS