Fix crash when a shard version didn't contain a shard.yml

[waj]

Versions without a spec will just be considered as if they don't have dependencies, but a warning will be printed if a shard is installed without a shard.yml.

For example, stefanwille/crystal-redis didn't have a shard.yml in the first versions. But it will now install without problems if latest versions are used:

$ shards install
Resolving dependencies
Installing redis (1.11.0)
Writing shard.lock

However, when a version without spec is selected it will print the warning:

$ shards install
Resolving dependencies
Shard "redis" version (1.0.1) doesn't have a shard.yml file
Installing redis (1.0.1)
Writing shard.lock

Fixes #346

[oprypin]

Please make a release with this, people in chat are running into this regularly.