don't install requirements.txt if environment.yml is present

The most likely cause of both being present is equivalent dependencies for two use cases,
not a single multi-stage conda environment.

If deps should be installed in conda with pip, they will be in `environment.yml` under `pip`.

repo2docker/detectors.py

@@ -554,7 +554,7 @@ class CondaBuildPack(BuildPack):
                 conda clean -tipsy
                 """
             ))
-        if os.path.exists('requirements.txt'):
+        elif os.path.exists('requirements.txt'):
             assembly_scripts.append((
                 '${NB_USER}',
                 'pip install --no-cache-dir -r requirements.txt'

tests/conda/requirements/README.rst

@@ -1,5 +1,12 @@
 Python - Mixed Requirements
 ---------------------------
 
-You can specify both a ``requirements.txt`` and an ``environment.yml`` file,
+An ``environment.yml`` takes precedence over ``requirements.txt``.
-and both of these will be used to build your environment.
+To install files into a conda environment with pip, use the ``pip`` key in ``environment.yml``:
+
+.. sourcecode:: yaml
+
+    dependencies:
+      - numpy
+      - pip:
+        - tornado

tests/conda/requirements/environment.yml

@@ -1,2 +1,4 @@
 dependencies:
   - numpy
+  - pip:
+    - simplejson

tests/conda/requirements/verify

@@ -4,4 +4,9 @@ import sys
 assert sys.version_info[:2] == (3, 6)
 
 import numpy
-import there
+try:
+    import there
+except ImportError:
+    print('ok')
+else:
+    raise Exception("'there' shouldn't have been installed from requirements.txt")