databases/rubygem-influxdb: update to 0.8.1

Changelog:
https://github.com/influxdata/influxdb-ruby/blob/master/CHANGELOG.md

v0.8.1, release 2021-02-17

    Ensure workers can send data popped off the queue at shutdown (#239, @onlynone)
    Add support for special characters in password when using url keyword (#242, @swistak35)
    Add Ruby 3.0 support (#249, @dentarg, @paul and @track8)
    Support characters that need quoting for usernames and database names (#248, @estheruary)

v0.8.0, released 2020-02-05

    Allow dropping of specific series from specific DBs (#233, @cantino)
    Add support for MRI 2.7 (#235, @jeffkowalski)
    Raise a LineProtocolError if attempting to write empty values as field set is required. This adds descriptive feedback to the error "unable to parse '{series},{tags} ': invalid field format" (#236, @todtb)
    Add support for configuring HTTP Proxy (#238, @epchris)