chore: release 1.13.1-SNAPSHOT (#51)

* updated versions.txt [ci skip]

* updated pom.xml [ci skip]

Author: release-please[bot]

pom.xml

@@ -4,7 +4,7 @@
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>
   <artifactId>google-cloud-spanner-jdbc</artifactId>
-  <version>1.13.0</version><!-- {x-version-update:google-cloud-spanner-jdbc:current} -->
+  <version>1.13.1-SNAPSHOT</version><!-- {x-version-update:google-cloud-spanner-jdbc:current} -->
   <packaging>jar</packaging>
   <name>Google Cloud Spanner JDBC</name>
   <url>https://github.com/googleapis/google-cloud-java/tree/master/google-cloud-clients/google-cloud-contrib/google-cloud-spanner-jdbc</url>

versions.txt

@@ -1,4 +1,4 @@
 # Format:
 # module:released-version:current-version
 
-google-cloud-spanner-jdbc:1.13.0:1.13.0
+google-cloud-spanner-jdbc:1.13.0:1.13.1-SNAPSHOT