Return an error when users tries to use below go 1.5.1 version for dl and use commands

[olimpias]

Currently, when you try to use go 1.4.x version, gvm is not working. We should not support the below go 1.5.1 version.

Example Command:

gvm use 1.4.1

Expected Result:

gvm version 0.1.1
Below go1.5.x is not supported

We should add a validation method here and write a unit test for it.

New validation method should be used here and here