Upgrade Guava 25.0-jre -> 26.0-jre (#31)

See:
- https://github.com/google/guava/releases/tag/v25.1
- https://github.com/google/guava/releases/tag/v26.0
- https://google.github.io/guava/releases/25.1-jre/api/diffs/
- https://google.github.io/guava/releases/26.0-jre/api/diffs/
- google/guava@v25.0...v26.0

pom.xml

@@ -137,7 +137,7 @@
             <dependency>
                 <groupId>com.google.guava</groupId>
                 <artifactId>guava</artifactId>
-                <version>25.0-jre</version>
+                <version>26.0-jre</version>
             </dependency>
             <dependency>
                 <groupId>com.google.guava</groupId>