chore: release 1.15.1-SNAPSHOT (googleapis#101)

🤖 I have created a release \*beep\* \*boop\* 
---
### Updating meta-information for bleeding-edge SNAPSHOT release.
---


This PR was generated with [Release Please](https://github.com/googleapis/release-please).

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>1.15.0</version><!-- {x-version-update:google-cloud-spanner-jdbc:current} -->
+  <version>1.15.1-SNAPSHOT</version><!-- {x-version-update:google-cloud-spanner-jdbc:current} -->
   <packaging>jar</packaging>
   <name>Google Cloud Spanner JDBC</name>
   <url>https://github.com/googleapis/java-spanner-jdbc</url>

versions.txt

@@ -1,4 +1,4 @@
 # Format:
 # module:released-version:current-version
 
-google-cloud-spanner-jdbc:1.15.0:1.15.0
+google-cloud-spanner-jdbc:1.15.0:1.15.1-SNAPSHOT