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

@@ -110,11 +110,16 @@ function main (opts) {
 
   function readJson (jsonPath, name, ignoreMissing) {
     return fs.readFileAsync(path.join(jsonPath, name), 'utf8')
-    .then(str => JSON.parse(str))
+    .then(stripBOM)
+    .then(JSON.parse)
     .catch({code: 'ENOENT'}, err => {
       if (!ignoreMissing) {
         throw err
       }
     })
   }
+
+  function stripBOM (str) {
+    return str.replace(/^\uFEFF/, '')
+  }
 }