$ charm build
build: Composing into /vagrant/charms
build: Added unexpected file, should be in a base layer: layers.yaml
Traceback (most recent call last):
  File "/usr/local/bin/charm-build", line 9, in <module>
    load_entry_point('charm-tools==1.8.0', 'console_scripts', 'charm-build')()
  File "/usr/local/lib/python2.7/dist-packages/charmtools/build/__init__.py", line 520, in main
    build()
  File "/usr/local/lib/python2.7/dist-packages/charmtools/build/__init__.py", line 421, in __call__
    self.validate()
  File "/usr/local/lib/python2.7/dist-packages/charmtools/build/__init__.py", line 412, in validate
    "Unable to continue due to unexpected modifications "
ValueError: Unable to continue due to unexpected modifications (try --force)