Remove old versions of gems... gem cleanup is not doing it

Question

This is the list of gems of my system after issuing a sudo gem cleanup command:

activesupport (4.2.6)
addressable (2.5.0)
babosa (1.0.2)
bigdecimal (1.3.1, default: 1.2.0)
bundler (1.14.4)
capybara (2.7.0)
CFPropertyList (2.3.5)
claide (1.0.2)
cliver (0.3.2)
cocoapods (1.2.1)
cocoapods-core (1.2.1)
cocoapods-deintegrate (1.0.1)
cocoapods-downloader (1.1.3)
cocoapods-plugins (1.0.0)
cocoapods-search (1.0.0)
cocoapods-stats (1.0.0)
cocoapods-trunk (1.2.0)
cocoapods-try (1.1.0)
coderay (1.1.1)
colored (1.2)
colored2 (3.1.2)
commander (4.4.3)
concurrent-ruby (1.0.4)
credentials_manager (1.0.0)
declarative (0.0.9)
declarative-option (0.1.0)
deliver (2.0.1)
domain_name (0.5.20161129)
dotenv (2.2.0)
escape (0.0.4)
excon (0.55.0)
faraday (0.11.0)
faraday-cookie_jar (0.0.6)
faraday_middleware (0.11.0.1)
fastimage (2.0.1)
fastlane (2.15.1)
fastlane_core (1.0.0)
fourflusher (2.0.1)
fuzzy_match (2.1.0, 2.0.4)
gh_inspector (1.0.3)
google-api-client (0.10.0, 0.9.28)
googleauth (0.5.1)
highline (1.7.8)
http-cookie (1.0.3)
httpclient (2.8.3)
hurley (0.2)
i18n (0.8.0)
io-console (0.4.6, default: 0.4.2)
json (2.0.3, 1.8.3, default: 1.7.7)
json_pure (2.0.3)
jwt (1.5.6)
little-plugger (1.1.4)
logging (2.1.0)
memoist (0.15.0)
method_source (0.8.2)
mime-types (3.1)
mime-types-data (3.2016.0521)
mini_magick (4.6.1, 4.5.1)
mini_portile (0.6.2)
mini_portile2 (2.1.0)
minitest (5.10.1, 5.8.4, 5.5.1, default: 4.3.2)
molinillo (0.5.7)
multi_json (1.12.1)
multi_xml (0.6.0)
multipart-post (2.0.0)
nanaimo (0.2.3)
nap (1.1.0)
netrc (0.11.0, 0.7.8)
open4 (1.3.4)
os (0.9.6)
pdf-core (0.6.1)
phantomjs (2.1.1.0)
plist (3.2.0)
poltergeist (1.13.0)
power_assert (1.0.1)
prawn (2.1.0)
pry (0.10.4)
psych (2.2.2, default: 2.0.0)
public_suffix (2.0.5)
rack (1.6.4)
rack-test (0.6.3)
rake (12.0.0, 10.4.2, default: 0.9.6)
rdoc (4.2.2, default: 4.0.0)
representable (3.0.3, 2.3.0)
retriable (3.0.0, 2.1.0)
rouge (2.0.7, 1.10.1)
ruby-macho (1.1.0)
rubygems-update (2.6.10)
rubyzip (1.2.1, 1.1.7)
security (0.1.3)
sentry-raven (2.3.0)
signet (0.7.3)
slack-notifier (2.0.0, 1.5.1)
slop (4.4.1, 3.6.0)
spaceship (1.0.0)
terminal-notifier (1.7.1)
terminal-table (1.7.3)
test-unit (3.2.3, default: 2.0.0.0)
thread_safe (0.3.5)
ttfunk (1.4.0)
tty-screen (0.5.0)
tzinfo (1.2.2)
uber (0.1.0, 0.0.15)
unf (0.1.4)
unf_ext (0.0.7.2)
unicode-display_width (1.1.3)
websocket-driver (0.6.5)
websocket-extensions (0.1.2)
word_wrap (1.0.0)
wwdcdownloader (20.17.0)
xcodeproj (1.5.0)
xcpretty (0.2.4)
xcpretty-travis-formatter (0.0.4)
xpath (2.0.0)

as you can see, several gems, like this one

minitest (5.10.1, 5.8.4, 5.5.1, default: 4.3.2)

have several versions installed and this default pointing to the oldest versions.

I want to delete all these versions keeping just the latest one and assigning the default to that latest one. sudo gem cleanup or even sudo gem cleanup rjb is not doing it.

Any suggestions? thanks.