Allow specifying requirements.txt with environment.yml

They both get installed.

repo2docker/detectors.py

@@ -552,6 +552,11 @@ class CondaBuildPack(BuildPack):
                 conda clean -tipsy
                 """
             ))
+        if os.path.exists('requirements.txt'):
+            assembly_scripts.append((
+                '${NB_USER}',
+                'pip install --no-cache-dir -r requirements.txt'
+            ))
         return assembly_scripts
 
     def detect(self):

tests/conda/requirements/environment.yml

@@ -0,0 +1,2 @@
+dependencies:
+  - numpy

tests/conda/requirements/requirements.txt

@@ -0,0 +1 @@
+there

tests/conda/requirements/verify

@@ -0,0 +1,7 @@
+#!/usr/bin/env python
+import sys
+
+assert sys.version_info[:2] == (3, 6)
+
+import numpy
+import there