Attempt to fix build.

Apparently pip now requires --allow-external to install argparse.
Another build fix, PIL requires --allow-unverified.
added YAMLLoader "if" directive support
Merge branch 'master' of https://github.com/miracle2k/webassets into feature/conditional-yaml
Merge branch 'feature/conditional-yaml' into pypi/webassets2

Conflicts:
	src/webassets/loaders.py
bumped version