cucumber step: unnecessary wildcard

While building out another project using fix_spec, I was getting `Cucumber::Ambiguous` conflicts with the a step similar to:
```cucumber
Given I create the following NOT_FIX message:
"""
...
"""
```
because we are using the regex `(?:fix|FIX|(.*))` ([link](https://github.com/connamara/fix_spec/blob/master/lib/fix_spec/builder.rb#L26)) for a fix related cucumber step. Note the wildcard, which causes the conflict. We're using this to pre populate Begin Strings with values like `FIX.4.2`. But using a free-form text wildcard isn't necessary here. It is sufficient to just permit the minimum possible values with `(?:fix|FIX|(FIX\.\d+\.\d+))`. This will help prevent potential user errors, and allow others to define their own steps for creating messages that aren't FIX related (like the example above)

Note this will require a version bump.