Upgrade errorprone-slf4j 0.1.13 -> 0.1.15 (#218)

See:
- https://github.com/KengoTODA/errorprone-slf4j/releases/tag/v0.1.14
- https://github.com/KengoTODA/errorprone-slf4j/releases/tag/v0.1.15
- KengoTODA/errorprone-slf4j@v0.1.13...v0.1.15

pom.xml

@@ -146,7 +146,7 @@
         <version.error-prone>${version.error-prone-orig}</version.error-prone>
         <version.error-prone-fork>v${version.error-prone-orig}-picnic-2</version.error-prone-fork>
         <version.error-prone-orig>2.14.0</version.error-prone-orig>
-        <version.error-prone-slf4j>0.1.13</version.error-prone-slf4j>
+        <version.error-prone-slf4j>0.1.15</version.error-prone-slf4j>
         <version.guava-beta-checker>1.0</version.guava-beta-checker>
         <version.jdk>11</version.jdk>
         <version.maven>3.8.6</version.maven>