Makefile: [fix] allow minor releases for RPMs

Previously, incrementing the Release number in stb-tester.spec.in to
anything other than 1 would cause the srpm make target to break because
the name of the produced RPM would not match that which was hardcoded to
the "src_rpm" variable.

This commit fixes the issue by substituting the minor release into the
RPM spec file in the same way that the major version is. Users wishing to
create minor releases can now either hardcode the increment into the
Makefile by changing the assignment to the `RELEASE` variable or simply
assign it on the `make` command line, e.g. `make rpm RELEASE=3`.
Makefile: [fix] do not hard code the Fedora release for the RPM

Previously, any created RPM would be named for Fedora 20. Now, the created
RPM targets the installed (and active) OS version. Note that `rpm -E %dist`
returns (for example) ".fc20" *including* the leading dot.
Makefile: add `sudo` specifically for yum-builddep in `make rpm`

`yum-builddep` requires super user privileges, which meant that users
would have to run `make rpm` as root user in order for it to succeed. This
commit adds a `sudo` specifically for the `yum-builddep` call to avoid
running the whole make recipe (and any of its dependencies) as the root
user, which would, for example, create the "VERSION" file with root privs.