Use `exec` instead of `spawn` to run node-gyp.

Without this change, execSync fails to install because `spawn` cannot find
`node-gyp`, despite it being on the `PATH` of the shell environment I'm
trying to install it from (which is the "Node.js Command Prompt" on
Windows).

`spawn` creates a new process, whereas `exec` executes the command in the
shell. By switching to `exec`, Node will run the `node-gyp` command in the
same shell environment that I launched `install.js` from, resulting in a
successful install.