3rd party gem cmds should be executed in the context of bundle exec

## motivation

When developing new features in the xamarin test cloud and calabash-ios gems, it is important that the correct gem versions are use when submitting to the XTC.

In the past, I have been reluctant to enforce the use of bundler.  Recently, I had some mis-steps because I was not calling some 3rd party gem commands in the context `bundle exec`.

Briar now requires bundler.