add version specifiers to pandas and python requirements

This fixes a linter complaint ("noarch: python recipes are required to have a lower bound on the python version") that's showing up in PRs like conda-forge#6.

I'm not sure if the python 3.2 lower bound is actually correct. The fredapi package's contents itself do not specify a minimum Python version in the documentation or metadata, as far as I can see. I selected 3.2 because that's the minimum version required by pandas 0.15, and the fredapi package does declare a 'pandas>=0.15' dependency in its requirements.txt.

recipe/meta.yaml

@@ -17,10 +17,10 @@ build:
 requirements:
   host:
     - pip
-    - python
+    - python >=3.2
   run:
-    - pandas
-    - python
+    - pandas >=0.15
+    - python >=3.2
 
 test:
   imports: