Update proguard-rules.pro for android-23

[grantland]

android-23 removed some APIs that our SDK uses internally. Luckily,
we only conditionally use these APIs on older devices so this is
a non-issue for our SDK running on android-23.

This change adds instructions to ignore warnings from proguard stating
that these APIs do not exist when using compileSdkVersion = 23 since
we already have checks in place.

Test Plan:

Change ParseStarterProject's build.gradle:

android {
    compileSdkVersion 23
    useLibrary `org.apache.http.legacy`

    // ...

    buildTypes {
        release {
            minifyEnabled true

            // ...
        }
    }
}

Resolves #231

[facebook-github-bot]

By analyzing the blame information on this pull request, we identified @grantland to be a potential reviewer.

[grantland]

ParseApacheHttpClient which uses missing android.net.SSLCertificateSocketFactory only gets used on < android-19: https://github.com/ParsePlatform/Parse-SDK-Android/blob/master/Parse/src/main/java/com/parse/ParseHttpClient.java#L51

NotificationCompatImplBase which uses missing android.app.Notification.setLatestEventInfo only gets used on < android-14: https://github.com/ParsePlatform/Parse-SDK-Android/blob/master/Parse/src/main/java/com/parse/NotificationCompat.java#L103

[wangmengyan95]

LGTM

[facebook-github-bot]

@grantland updated the pull request.