PARQUET-375: Update current release version in README.md
PARQUET-375: Update current release version in README.md

Current README.md was still pointing to release 1.7.0

Author: Luciano Resende <lresende@apache.org>

Closes #260 from lresende/update-release-version and squashes the following commits:

f7a6964 [Luciano Resende] Update current release version in README.md

winningsix/incubator-parquet-mr,apache/parquet-mr,SaintBacchus/parquet-mr,HyukjinKwon/parquet-mr-1,HyukjinKwon/parquet-mr-1,HyukjinKwon/parquet-mr,saucam/incubator-parquet-mr,coughman/incubator-parquet-mr,SinghAsDev/parquet-mr,SaintBacchus/parquet-mr,nitin2goyal/parquet-mr-1,SinghAsDev/parquet-mr,saucam/incubator-parquet-mr,nitin2goyal/parquet-mr-1,coughman/incubator-parquet-mr,apache/parquet-mr,coughman/incubator-parquet-mr,rdblue/parquet-mr,laurentgo/parquet-mr,laurentgo/parquet-mr,rdblue/parquet-mr,saucam/incubator-parquet-mr,rdblue/parquet-mr,winningsix/incubator-parquet-mr,jaltekruse/parquet-mr-1,nezihyigitbasi-nflx/parquet-mr,laurentgo/parquet-mr,winningsix/incubator-parquet-mr,cchang738/parquet-mr,nitin2goyal/parquet-mr-1,piyushnarang/parquet-mr,HyukjinKwon/parquet-mr-1,nezihyigitbasi-nflx/parquet-mr,nezihyigitbasi-nflx/parquet-mr,piyushnarang/parquet-mr,jaltekruse/parquet-mr-1,jaltekruse/parquet-mr-1,cchang738/parquet-mr,SaintBacchus/parquet-mr,piyushnarang/parquet-mr,SinghAsDev/parquet-mr,HyukjinKwon/parquet-mr,apache/parquet-mr,HyukjinKwon/parquet-mr,cchang738/parquet-mr