Update CI targets

Drop support for R15 because old and doesn't compile on most
of my platforms anymore.

Makefile

@@ -8,11 +8,11 @@ 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 \
+CI_OTP ?= \
 	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-17.5.6.6 \
-	OTP-18.0.3 OTP-18.1.5 OTP-18.2.1
+	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-18.2.4.1 OTP-18.3.4.4 \
+	OTP-19.0.7 OTP-19.1.2
 
 # Only test on the most recent version on public CI services.
 ifdef CI