execSync crashes process with custom shell on Windows

[petekanev]

Changes introduced with PR #118 cause a crash on Windows when a non default shell is used (git bash).

Mine is located in C:/Program Files/Git/.... for example, and the path contains whitespaces, resulting in the following error output:

[NativeScriptCli] execute: tns --version
Error: Command failed: tns --version
/c/Program: Files\Git\usr\bin\bash: No such file or directory
Error: NativeScript CLI not found. Use 'nativescript.tnsPath' workspace setting to explicitly set the absolute path to the NativeScript CLI.

It is a known node.js issue - nodejs/node#6803

[petekanev]

I propose a fix for Windows that will always default to the cmd.exe (just as it has been before 0.7).

I have not tested whether having a whitespace in the path to the default shell on a Unix OS will produce the same node error, though based on the original node issue - it should.