REGEX not matching all composer versions

## In which file did you encounter the issue?
File: `composer/tools/composer_dags.py`

## Did you change the file? If so, how?
No

## Describe the issue
The current regex expression does not work for environment versions that do not include minor and or patch

## Error
```
Traceback (most recent call last):
File "/home/rkillkelley/composer_dags.py", line 229, in
main(
File "/home/rkillkelley/composer_dags.py", line 168, in main
).groups()
AttributeError: 'NoneType' object has no attribute 'groups'
```