CLI installs beta versions of runtimes in case they are published in npm

[rosen-vladimirov]

In case there are beta versions of runtimes published in npm, tns platform add <platform> installs latest beta version instead of the one marked as latest. When version is not specified, CLI *must install the latest compatible version. This means:

• latest patch version matching CLI's version. For example, in case CLI 's version is 1.6.1, CLI should check the version marked as latest in npm. If it is 1.6.x, CLI should use it. In case it's minor version is different, CLI should list all versions and install the latest 1.6.x (no beta versions allowed).
• In case user specifies a version: tns platform add <platform>@<version>, CLI should respect it. This is the way to install beta versions.