Build fail if HERON_BUILD_VERSION environment variable is not set

[nwangtw]

Following the instructions below to build Heron:

https://apache.github.io/incubator-heron/docs/developers/compiling/compiling/

If HERON_BUILD_VERSION is not set, the following error will be triggered:

*ERROR: *Process exited with status 1: Process exited with status 1

scripts/release/status.sh: line 62: HERON_BUILD_VERSION: unbound variable

[nwangtw]

This happens only when the source code is downloaded and decompressed.

The fix is here: https://github.com/apache/incubator-heron/pull/3245