Dedupe boot scripts

Remove duplicated entries in the array of boot scripts to run.
Because the bootDirs defaults to process.cwd(), if the user manually
includes scripts in the same directory, without specifying a bootDir,
those scripts will be included multiple times.
Merge pull request #72 from esatterwhite/lb-64-multiple-invocation-same-boot-script

Ensure to dedupe boot scripts array before running.

Closes #72
Fixes #64