Race condition in repackaged Base64 breaks auths at random

After updating our auth0 libs to pull in some important fixes, we started having random authentication failures that would not reproduce.

Turns out that `JWTVerifier.decodeAndParse()` was failing with IOException from time to time.

Diagnosis: the base64 decoder used is not thread safe. For some reason you repackaged a buggy commons-codec.

PS: This took me almost a whole day to nail down and it's a blocker for us. 

Proof of concept:

```
import com.auth0.jwt.internal.org.apache.commons.codec.binary.Base64;

public class Poc {

  public static void main(String... args) throws Exception {

    final int threads = 2;

    String[] tokens = new String[] {
        "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",
        "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"
    };

    final Base64 decoder = new Base64(true);

    for (int i = 0; i < threads; i++) {

      final String b64String = tokens[i % tokens.length];
      final String expected = new String(decoder.decode(b64String), "UTF-8"); // no concurrency here

      (new Thread() {

        @Override
        public void run() {
          System.err.println("Starting thread");
          try {
            while (true) {
              final String jsonString = new String(decoder.decode(b64String), "UTF-8");
              if (!expected.equals(jsonString)) {
                System.err.println("BUG: got <" +  jsonString + ">, expected + <" + expected + ">");
                break;
              }
            }
          } catch (Exception e) {
            e.printStackTrace();
          }
          System.exit(1);
        }

      }).start();

    }
  }
}

```

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