Bugfix for requirements.txt options ordering (#237)

Fixes #236

@vickeryj Can you please try this?

```
rm -Rf node_modules/serverless-python-requirements
npm i github:andrewfarley/serverless-python-requirements#bug-fix-requirements-ordering
```

Author: AndrewFarley

lib/pip.js

@@ -267,7 +267,9 @@ function dockerPathForWin(options, path) {
 
 /** create a filtered requirements.txt without anything from noDeploy
  *  then remove all comments and empty lines, and sort the list which
- *  assist with matching the static cache
+ *  assist with matching the static cache.  The sorting will skip any
+ *  lines starting with -- as those are typically ordered at the
+ *  start of a file ( eg: --index-url / --extra-index-url )
  * @param {string} source requirements
  * @param {string} target requirements where results are written
  * @param {Object} options
@@ -277,14 +279,24 @@ function generateRequirementsFile(source, target, options) {
   const requirements = fse
     .readFileSync(source, { encoding: 'utf-8' })
     .split(/\r?\n/);
+  var prepend = [];
   const filteredRequirements = requirements.filter(req => {
     req = req.trim();
-    if (req.length == 0 || req[0] == '#') {
+    if (req.startsWith('#')) {
+      // Skip comments
+      return false;
+    } else if (req.startsWith('--')) {
+      // If we have options (prefixed with --) keep them for later
+      prepend.push(req);
       return false;
     }
     return !noDeploy.has(req.split(/[=<> \t]/)[0].trim());
   });
-  filteredRequirements.sort(); // Sort them alphabetically
+  filteredRequirements.sort(); // Sort remaining alphabetically
+  // Then prepend any options from above in the same order
+  for (let item of prepend.reverse()) {
+    filteredRequirements.unshift(item);
+  }
   fse.writeFileSync(target, filteredRequirements.join('\n'));
 }