upgrade to rxjava 2.2.15

Author: davidmoten

pom.xml

@@ -20,7 +20,7 @@
         <scm.url>scm:git:https://github.com/davidmoten/rxjava2-jdbc.git</scm.url>
         <maven.compiler.target>1.8</maven.compiler.target>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-        <rxjava.version>2.2.11</rxjava.version>
+        <rxjava.version>2.2.15</rxjava.version>
         <checkstyle.version>2.17</checkstyle.version>
         <findbugs.version>3.0.5</findbugs.version>
         <javadoc.version>3.0.0</javadoc.version>