build: Fix release build names. (#4263)

Currently due to the occurrence of 6 arguments from
`gen-ldflags.go` leads to a bug where the binaries
genenerated have wrong names.

As shown below.

```
If you want to build for all, Just press Enter: linux/amd64
-->    linux/amd64:github.com/minio/minio
$ ls release/linux-amd64/
[2017-05-04 23:08:51 PDT]  17MiB minio
[2017-05-04 23:08:51 PDT]  17MiB minio.2017-05-05T06:08:22Z
[2017-05-04 23:08:51 PDT]    76B minio.shasum
```

This PR fixes this issue by retaining the previous release
binary names.

```
If you want to build for all, Just press Enter: linux/amd64
-->    linux/amd64:github.com/minio/minio
$ ls release/linux-amd64/
[2017-05-04 23:08:51 PDT]  17MiB minio
[2017-05-04 23:08:51 PDT]  17MiB minio.RELEASE.2017-05-05T06-08-22Z
[2017-05-04 23:08:51 PDT]    76B minio.shasum
```

buildscripts/build.sh

@@ -5,7 +5,7 @@ _init() {
     LDFLAGS=$(go run buildscripts/gen-ldflags.go)
 
     # Extract release tag
-    release_tag=$(echo $LDFLAGS | awk {'print $4'} | cut -f2 -d=)
+    release_tag=$(echo $LDFLAGS | awk {'print $6'} | cut -f2 -d=)
 
     # Verify release tag.
     if [ -z "$release_tag" ]; then