Fix bug with vendored assets

`gem bump` tries to bump versions for any gemspecs in the existing path.

This causes errors and aborted bumps when you vendor gems into a local `vendor` directory.

```
λ 2.1.1 micro_flip> gem bump
Bumping micro_flip from 0.1.1 to version 0.1.2
Bumping method_source from 0.1.1 to version 0.1.2
The following paths are ignored by one of your .gitignore files:
vendor/ruby/2.1.0/gems/method_source-0.8.2/lib/method_source/version.rb
Use -f if you really want to add them.
fatal: no files added
The task `bump` could not be completed. Subsequent tasks will not be attempted.
```

The desired behavior is that it would bump `micro_flip` gem that I'm working on, but it's also trying to bump all gems that are in the `micro_flip/vendor/` directory.

I fixed this in a very targeted way by excluding any assets in the `vendor` directory.  

If you like the patch, let me know and I'll add testing before it's merged in.