Travis CI build job is failing due to error parsing Gemfile

[mloskot]

Lately, e.g. https://travis-ci.org/Azure/vagrant-azure/builds/608451607, the jobs have been failing:

Your Gemfile lists the gem vagrant-azure (>= 0) more than once.
You should probably keep only one of them.
Remove any duplicate entries and specify the gem only once (per group).
While it's not a problem now, it could cause errors if you change the version of one of them later.

[!] There was an error parsing `Gemfile`: You cannot specify the same gem twice coming from different sources.
You specified that vagrant-azure (>= 0) should come from source at `.` and source at `.`
. Bundler cannot continue.

 #  from /home/travis/build/Azure/vagrant-azure/Gemfile:17
 #  -------------------------------------------
 #  group :plugins do
 >    gem "vagrant-azure", path: "."
 #  end
 #  -------------------------------------------

Could anyone fix it? Seems like Ruby update may be required...I know nothing about Ruby myself.