Bump postgresql from 42.2.18 to 42.2.20 in /modules/postgresql

Bumps [postgresql](https://github.com/pgjdbc/pgjdbc) from 42.2.18 to 42.2.20.
- [Release notes](https://github.com/pgjdbc/pgjdbc/releases)
- [Changelog](https://github.com/pgjdbc/pgjdbc/blob/REL42.2.20/CHANGELOG.md)
- [Commits](pgjdbc/pgjdbc@REL42.2.18...REL42.2.20)

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

Author: dependabot[bot]

modules/postgresql/build.gradle

@@ -11,7 +11,7 @@ dependencies {
 
     testCompile project(':jdbc-test')
     testCompile project(':test-support')
-    testCompile 'org.postgresql:postgresql:42.2.18'
+    testCompile 'org.postgresql:postgresql:42.2.20'
 
     testCompile testFixtures(project(':r2dbc'))
     testCompile 'io.r2dbc:r2dbc-postgresql:0.8.5.RELEASE'