fix: strip BOM when reading JSON files

fs.readFile* don't strip the BOM header, even when specifying 'utf8' as
the encoding, and JSON.parse() doesn't handle it either. There are
technically a bunch of BOM indicators, but this is the one seen most
commonly and actually appears in a number of package.json files in the
wild.

See nodejs/node#6924, nodejs/node#3040 for background.

index.js

@@ -138,10 +138,17 @@ module.exports = Installer
 
 function readJson (jsonPath, name, ignoreMissing) {
   return readFileAsync(path.join(jsonPath, name), 'utf8')
-  .then(str => JSON.parse(str))
+  .then(str => JSON.parse(stripBOM(str)))
   .catch({code: 'ENOENT'}, err => {
     if (!ignoreMissing) {
       throw err
     }
   })
 }
+
+function stripBOM (str) {
+  if (str.charCodeAt(0) === 0xFEFF) {
+    str = str.slice(1)
+  }
+  return str
+}