updating gradle to version 3.1

CHANGELOG.md

 Upcoming version:
 -----------------
+  - updating gradle to version 3.1 (Nikolaus Krismer)
   - fixed minor problems with spaces in deploy directory (Nikolaus Krismer)
   - changes to match new argument naming in isochrone-tools project (Nikolaus Krismer)
   - updating neo4j to latest version (Nikolaus Krismer)

gradle/wrapper/gradle-wrapper.properties

-#Fri Aug 19 10:17:45 CEST 2016
+#Wed Oct 05 21:00:08 CEST 2016
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-3.0-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-3.1-bin.zip