Confusing VERSION file on master branch

[StevenYCChou]

VERSION file on master branch is confusing. For example,
fc1c9e2 is included in release 0.4.2, but the VERSION file still shows as 0.4.0.

#81 pointed out VERSION file is used to build the binary. Here is an example when you show the version of the binary:

$ ./stackdriver-prometheus-sidecar --version
prometheus, version 0.4.0 (branch: master, revision: b9b9b5facf6dc90e233ce93927960d5aee4bf360)
  build user:       [REDACTED]
  build date:       20190517-14:47:05
  go version:       go1.12

I purpose that we use latest in VERSION on master, so when anyone build the binary from master, this user knows that it's not on a specific named version.