Bump postgresql from 42.2.14 to 42.2.15 in /modules/postgresql (#3113)

Bumps [postgresql](https://github.com/pgjdbc/pgjdbc) from 42.2.14 to 42.2.15.
- [Release notes](https://github.com/pgjdbc/pgjdbc/releases)
- [Changelog](https://github.com/pgjdbc/pgjdbc/blob/master/CHANGELOG.md)
- [Commits](pgjdbc/pgjdbc@REL42.2.14...REL42.2.15)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

modules/postgresql/build.gradle

@@ -11,7 +11,7 @@ dependencies {
 
     testCompile project(':jdbc-test')
     testCompile project(':test-support')
-    testCompile 'org.postgresql:postgresql:42.2.14'
+    testCompile 'org.postgresql:postgresql:42.2.15'
 
     testCompile testFixtures(project(':r2dbc'))
     testCompile 'io.r2dbc:r2dbc-postgresql:0.8.3.RELEASE'