fix: avoid using deprecated Buffer constructor (#94)

* fix: avoid using deprecated Buffer constructor

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

* readShebang: name reused inline constant

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

* readShebang: Buffer API compat for old Node.js

To be reverted when dropping outdated Node.js versions support.

lib/util/readShebang.js

@@ -5,12 +5,23 @@ const shebangCommand = require('shebang-command');
 
 function readShebang(command) {
     // Read the first 150 bytes from the file
+    const size = 150;
+
+    let buffer;
+    if (Buffer.alloc) {
+        // Node.js v4.5+ / v5.10+
+        buffer = Buffer.alloc(size);
+    } else {
+        // Old Node.js API
+        buffer = new Buffer(size);
+        buffer.fill(0); // zero-fill
+    }
+
     let fd;
-    const buffer = new Buffer(150);
 
     try {
         fd = fs.openSync(command, 'r');
-        fs.readSync(fd, buffer, 0, 150, 0);
+        fs.readSync(fd, buffer, 0, size, 0);
         fs.closeSync(fd);
     } catch (e) { /* Empty */ }