fix parsing of dbt vars in bigquery

[bendiktv2]

This closes #261.

This has been tested with running all three variants of passing variables to DBT from the CLI:

dbt run --vars '{"key": "value", "date": 20180101}'
dbt run --vars '{key: value, date: 20180101}'
dbt run --vars 'key: value'

[minhnhat992]

appreciate if this is could be looked at/merged, this would be really helpful for my works!

[glsdown]

Thanks for adding this fix, and apologies it has taken so long to get back to you.