Use verbosity=Verbosity.VERBOSE for io.write_line

[shawwn]

This PR ensures that poetry version -s prints the project version number (and nothing else).

Shell scripts often use poetry version -s to get the current project version number. However, poetry-version-plugin breaks this idiom by printing out extra log messages:

$ poetry version -s
poetry-version-plugin: Using __init__.py file at sgap/__init__.py for dynamic version
poetry-version-plugin: Setting package dynamic version to __version__ variable from __init__.py: 0.1.0
0.1.0

After merging this PR:

• poetry version -s will print just the version number (e.g. 0.1.0)
• poetry version -s -v will print the extra log messages (since --verbose is specified)

Closes #12.