honor escaped newlines in requirements.txt

fixes #261

Author: dschep

lib/pip.js

@@ -297,6 +297,7 @@ function generateRequirementsFile(source, target, options) {
   const noDeploy = new Set(options.noDeploy || []);
   const requirements = fse
     .readFileSync(source, { encoding: 'utf-8' })
+    .replace(/\\\n/g, ' ')
     .split(/\r?\n/);
   var prepend = [];
   const filteredRequirements = requirements.filter(req => {