Upgrade commons-net to 3.9.0 to address CVE-2021-37533

This newer version addresses CVE-2021-37533 which prevents issues for users who use this older version as a transitive nrjavaserial dependency.

For release notes, see:

https://commons.apache.org/proper/commons-net/changes-report.html

Fixes #237

build.gradle

@@ -42,7 +42,7 @@ repositories {
 dependencies {
  compile fileTree(dir: 'libs', includes: ['*.jar'])
  testCompile 'junit:junit:4.12'
- compile 'commons-net:commons-net:3.7.2'
+ compile 'commons-net:commons-net:3.9.0'
  compileOnly 'net.java.dev.jna:jna:4.4.0'
  compileOnly 'net.java.dev.jna:jna-platform:4.4.0'
 }