Put less Erlang versions to make CircleCI happy

Makefile

@@ -14,8 +14,7 @@ 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
 
 # Standard targets.