Buffer.alloc() is supported on Node.js >= 4.5.0, and allocates a zero-filled
buffer on all supported versions.

This solves two issues:
 * Buffer() constructor (aka 'new Buffer()') is deprecated, so avoid using it
 * On 4.x and 6.x, Buffer(number) is not zero-filled, so this code was
   allocating an uninitialized Buffer when file length was less than 150, so
   the behaviour of readShebang() was actually undefined in that case (as in
   could have returned anything, depending on the uninitialized memory chunk).

Refs: https://nodejs.org/api/deprecations.html#deprecations_dep0005_buffer_constructor

'150' was used two times, it deserves a name.
It's the size of the pre-read buffer.

To be reverted when dropping outdated Node.js versions support.