Remove Gradle propdeps plugin

This commit removes the propdeps plugin.
"provided" configurations are replaced with "compileOnly", "testCompile"
configurations.
The "optional" configuration supported by propdeps is now replaced by a
local Gradle plugin defining a specific configuration for that.

As a result of that change, optional/provided dependencies are not
published with the POMs anymore.

spring-binding/spring-binding.gradle

@@ -4,6 +4,7 @@ dependencies {
 	compile("org.springframework:spring-context")
 
 	compileOnly("javax.el:javax.el-api")
+
 	testCompile("org.junit.jupiter:junit-jupiter")
 	testCompile("org.hamcrest:hamcrest")
 	testCompile("org.easymock:easymock")