Update Excon version to use >= 0.46

[yitsushi]

with older version of Excon, Docker responses with '400 Bad Request'

If you install this gem now, it's not a problem, but if you installed it before Excon=0.46
then docker-api gem installed excon and later updates don't require an update on excon.

Original issue: #381

Related issue: excon/excon#547

[yitsushi]

About "All checks have failed":
There were 4 failed job. All of them was "gpgkeys: key xxxx can't be retrieved" and because of this docker install was failed. Where docker install was successful, tests could run without error.

[tlunter]

I won't force people to update excon unless they are hitting this problem. Excon is used by lots of other gems. It's already noted in the README.

[yitsushi]

A question: If anyone uses old version of this gem and want to update it. Is there any way that they don't have to update excon? I mean with new docker and new docker-api gem, I could not find on 7 servers when I don't have to update excon also.

(if they don't want to update, they are not affected with this but they don't get the now gemspec also)