fix deprecation warnings from org.apache.http.Header

[klaeufer]

/Users/laufer/Dropbox/Home/Work/cs313/primenumbers-android-java/app/src/main/java/edu/luc/etl/cs313/android/primechecker/android/PrimeCheckerRemoteTask.java:12: warning: [deprecation] Header in org.apache.http has been deprecated
import org.apache.http.Header;
^
/Users/laufer/Dropbox/Home/Work/cs313/primenumbers-android-java/app/src/main/java/edu/luc/etl/cs313/android/primechecker/android/PrimeCheckerRemoteTask.java:49: warning: [deprecation] Header in org.apache.http has been deprecated
public void onSuccess(final int statusCode, final Header[] headers,
^
/Users/laufer/Dropbox/Home/Work/cs313/primenumbers-android-java/app/src/main/java/edu/luc/etl/cs313/android/primechecker/android/PrimeCheckerRemoteTask.java:55: warning: [deprecation] Header in org.apache.http has been deprecated
public void onFailure(final int statusCode, final Header[] headers,