Upgrade Maven project to Spock 2.0-M3, ... (#35)

* Sync Spock dependency versions
* Now that Spock no longer depends on 'groovy-all', for the database test it was
  necessary to add 'groovy-sql'.
* Make feature method names friendlier in unrolled DatabaseDrivenSpec
* Make IncludeExcludeExtensionSpec print messages
  This way the user can verify visually that in-/excludes actually work as
  expected.
* Upgrade Groovy 2.5.12 -> 2.5.13
* Slightly adjust README.adoc 
  Change passive style "all feedback is appreciated" to active style "we
  appreciate all feedback".

src/test/groovy/IncludeExcludeExtensionSpec.groovy

@@ -49,15 +49,18 @@ import spock.lang.Specification
 class IncludeExcludeExtensionSpec extends Specification {
   @Fast
   def "a fast method"() {
+    println "fast"
     expect: true
   }
 
   @Slow
   def "a slow method"() {
+    println "slow"
     expect: true
   }
 
   def "a neither fast nor slow method"() {
+    println "neither fast nor slow"
     expect: true
   }
 }