chore(ci): Update Ruby 2.1.8 in build matrix to 2.1.10
chore(ci): Update Ruby 2.1.8 in build matrix to 2.1.10

Ruby 2.1.10 is the last release of the 2.1.x series, so a better testing
benchmark.

bugsnag/bugsnag-ruby,bugsnag/bugsnag-ruby,bugsnag/bugsnag-ruby,bugsnag/bugsnag-ruby