Makefile depends on bash

[michielbdejong]

Since 0374913 it is no longer possible to build revad on Ubuntu, because it uses dash instead of bash as the shell, which doesn't support the [[ ... ]]] syntax.

[michielbdejong]

A workaround is to remove that first term from the build flags, so it becomes:

BUILD_FLAGS    = " -X main.gitCommit=$(GIT_COMMIT) -X main.version=$(VERSION) -X main.goVersion=$(GO_VERSION) -X main.buildDate=$(BUILD_DATE)"

That is what we will be using on our server, because we're not using the static linking feature there anyway.