Build script improvements

[kelloggm]

This PR contains some minor improvements to CryptoGuard's build scripts. There are three changes:

• fix gradlew so that ./gradlew clean build succeeds on MacOs. With what's currently checked into the repository, I encountered these errors:

  > ./gradlew clean build
  ./gradlew: line 158: syntax error near unexpected token `('
  ./gradlew: line 158: `methodWrapper splitJvmOpts() {'
  exit 2

• update the gradle version to 6.3. Both this and the previous point were accomplished by running gradle wrapper --gradle-version 6.3 from the root directory.
• make the dependency on javax.xml.bind explicit, because it has been removed in later versions of Java. This makes it possible to build CryptoGuard using a Java 11 JVM

[sazzad114]

Thanks for your contribution! :+1: