Fix issue when installing pyodbc

[kwigley]

resolves #122

before:

pip install ".[ODBC]"
Processing /Users/kwigley/workspace/dbt-spark
  WARNING: dbt-spark 0.18.1 does not provide the extra 'odbc'
# no pyodbc

after:

pip install ".[ODBC]"
Processing /Users/kwigley/workspace/dbt-spark
# no warning
...
Successfully installed ... pyodbc-4.0.30 ...

[jtcohen6]

I think our options are:

1. Update the README to say, "For v0.18.1, manually install pyodbc, this will be fixed in a future release."
2. Release this as v0.18.1.1 or v0.18.2. (We don't expect to release dbt-core v0.18.2.)

My personal preference is (2). In this repo, we have the opportunity to set precedents for a lot of other adapter plugins. I like the idea of adapter plugins mapping to dbt-core semver, certainly up to the minor version. I recall having previously decided that we'd prefer to maintain patch version equivalence, too, and should therefore add one more semantic number.

[gshank]

I have no strong opinions on how to handle this.