[loopback-boot-64] ensure to dedupe boot scripts array before running.

fix for [#64](https://github.com/strongloop/loopback-boot/issues/64)

Because the bootDirs defaults to process.cwd(), if the user manually
includes scripts in the same directory, with out specifying a bootDir,
those scripts will be included multiple times.

Use underscore.uniq to remove duplicate file paths.