Update CI OTP versions

And only use the last version on public CIs.

Makefile

@@ -7,11 +7,19 @@ PROJECT = ranch
 COMPILE_FIRST = ranch_transport
 CT_OPTS += -pa test -ct_hooks ranch_ct_hook []
 PLT_APPS = crypto public_key ssl
+
 CI_OTP = \
 	OTP_R15B01 OTP_R15B02 OTP_R15B03-1 \
 	OTP_R16B OTP_R16B01 OTP_R16B02 OTP_R16B03-1 \
-	OTP-17.0.2 OTP-17.1.2 OTP-17.2.2 OTP-17.3.4 OTP-17.4.1 OTP-17.5.6.2 \
-	OTP-18.0.2
+	OTP-17.0.2 OTP-17.1.2 OTP-17.2.2 OTP-17.3.4 OTP-17.4.1 OTP-17.5.6.6 \
+	OTP-18.0.3 OTP-18.1.5 OTP-18.2.1
+
+# Only test on the most recent version on public CI services.
+ifdef CI
+ifndef BUILDKITE
+CI_OTP := $(lastword $(CI_OTP))
+endif
+endif
 
 # Dependencies.