[build] Bump japicmp baseline to 3.1.3, allow for exclusions

This commit bumps the baseline to check against to the latest release,
3.1.3.RELEASE. Additionally, it bumps the japicmp plugin dependency so
that exclusions can be formulated, for exceptional situations where a
breaking change is necessary. Added a TODO to remember to do both things
after a release.

build.gradle

@@ -34,7 +34,7 @@ plugins {
   id 'org.asciidoctor.convert' version '1.5.6'
   id "me.champeau.gradle.jmh" version "0.4.4" apply false
   id "org.jetbrains.dokka" version "0.9.15"
-  id "me.champeau.gradle.japicmp" version "0.2.5"
+  id "me.champeau.gradle.japicmp" version "0.2.6"
 }
 
 apply from: "gradle/doc.gradle"
@@ -270,6 +270,9 @@ project('reactor-core') {
 	txtOutputFile = file("${project.buildDir}/reports/japi.txt")
 	ignoreMissingClasses = true
 	includeSynthetic = true
+
+	//TODO after a release, bump the gradle.properties baseline
+	//TODO after a release, remove the reactor-core exclusions below if any
   }
 
   javadoc {
@@ -438,6 +441,8 @@ project('reactor-test') {
 	txtOutputFile = file("${project.buildDir}/reports/japi.txt")
 	ignoreMissingClasses = true
 	includeSynthetic = true
+
+	//TODO after a release, remove the reactor-test exclusions below if any
   }
 
   javadoc {

gradle.properties

@@ -1,2 +1,2 @@
 version=3.1.4.BUILD-SNAPSHOT
-compatibleVersion=3.1.0.RELEASE
+compatibleVersion=3.1.3.RELEASE