Avoid internal OkHttp APIs

[yschimke]

Exception when app updates to 4.9.1 of OkHttp

     Caused by: java.lang.NoSuchFieldError: No static field UTF_8 of type Ljava/nio/charset/Charset; in class Lokhttp3/internal/Util; or its superclasses (declaration of 'okhttp3.internal.Util' appears in /data/app/~~jallv0v0SzQ_YjHgQaoPHQ==/com.gh4a.debug-trqypxRJBGXS5IfiP4jaNA==/base.apk!classes13.dex)
        at com.meisolsson.githubsdk.core.GitHubPaginationInterceptor.intercept(GitHubPaginationInterceptor.java:65)
        at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.kt:109)
        at okhttp3.internal.connection.RealCall.getResponseWithInterceptorChain$okhttp(RealCall.kt:201)
        at okhttp3.internal.connection.RealCall.execute(RealCall.kt:154)
        at retrofit2.OkHttpCall.execute(OkHttpCall.java:204)
        at retrofit2.adapter.rxjava2.CallExecuteObservable.subscribeActual(CallExecuteObservable.java:46)