Added support for electron beta versions

[WolverinDEV]

The current file regex does not support electron beta builds.
This pr should fix this :)

An example:

find_package(NodeJS)

set(NODEJS_URL "https://atom.io/download/atom-shell")
set(NODEJS_VERSION "v4.0.0-beta.7")
nodejs_init()

[computerquip-streamlabs]

The problem with this is beta is part of the version and its variable. For instance, there's nightly builds of electron that use "nightly" where this PR won't work anymore.