Fix #194 (#283)

Closes #194 

<strike>...no clue why git(hub) thinks there are 5 commits & 9 changed files (all those changes are already in the master; on which I rebased before branching)</strike>

lib/pip.js

@@ -381,6 +381,20 @@ function installRequirementsIfNeeded(
   // Our source requirements, under our service path, and our module path (if specified)
   const fileName = path.join(servicePath, modulePath, options.fileName);
 
+  // Skip requirements generation, if requirements file doesn't exist
+  if (options.usePipenv) {
+    if (
+      !fse.existsSync(path.join(servicePath, 'Pipfile')) &&
+      !fse.existsSync(fileName)
+    ) {
+      return false;
+    }
+  } else {
+    if (!fse.existsSync(fileName)) {
+      return false;
+    }
+  }
+
   // First, generate the requirements file to our local .serverless folder
   fse.ensureDirSync(path.join(servicePath, '.serverless'));
   const slsReqsTxt = path.join(servicePath, '.serverless', 'requirements.txt');