Bump maven-compiler-plugin from 3.6.1 to 3.8.1

Bumps [maven-compiler-plugin](https://github.com/apache/maven-compiler-plugin) from 3.6.1 to 3.8.1.
- [Release notes](https://github.com/apache/maven-compiler-plugin/releases)
- [Commits](apache/maven-compiler-plugin@maven-compiler-plugin-3.6.1...maven-compiler-plugin-3.8.1)

Author: dependabot-preview[bot]

pom.xml

@@ -37,7 +37,7 @@
         <maven-javadoc-plugin.version>2.10.4</maven-javadoc-plugin.version>
         <maven-cobertura-plugin.version>2.7</maven-cobertura-plugin.version>
         <maven-coveralls-plugin.version>4.3.0</maven-coveralls-plugin.version>
-        <maven-compiler-plugin.version>3.6.1</maven-compiler-plugin.version>
+        <maven-compiler-plugin.version>3.8.1</maven-compiler-plugin.version>
         <maven-gpg-plugin.version>1.6</maven-gpg-plugin.version>
         <maven-nexus-staging-plugin.version>1.6.8</maven-nexus-staging-plugin.version>
         <java.version>1.7</java.version>