BUG In Travis, restrict pandas version downloaded (#249)

Pandas dropped Python 3.4 support with v0.21, and our Python 3.4 tests have recently begun failing when they attempt to install the newest version. Our Travis tests were installing all of the optional packages in a separate line; move those to the dev-requirements file so that we can control which version is installed where. See also pandas-dev/pandas#20697 .

.travis.yml

@@ -7,7 +7,6 @@ python:
 install:
     - pip install --upgrade pip setuptools
     - pip install -r dev-requirements.txt
-    - pip install pandas scikit-learn scipy feather-format
     - pip install -e .
 env:
   global:

CHANGELOG.md

@@ -10,6 +10,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Include JSON files with pip distributions (#244)
 - Added flush to `civis_to_file` when passed a user-created buffer,
   ensuring the buffer contains the entire file when subsequently read.
+- Travis tests for Python 3.4 are now restricted to pandas<=0.20, the
+  last version which supported Python 3.4 (#249)
 
 ### Added
 - Added a utility function which can robustly split a Redshift schema name

dev-requirements.txt

@@ -6,4 +6,11 @@ pytest-cov>=2.4.0,==2.*
 vcrpy>=1.11.0,<=1.11.99
 vcrpy-unittest==0.1.6
 sphinx_rtd_theme>=0.2.4,<1.0.0
-numpydoc>=0.7.0,<1.0.0
+numpydoc>=0.7.0,<1.0.0
+feather-format
+numpy
+pandas; python_version >= '3.5'
+pandas<0.21; python_version == '3.4'
+pandas<=0.23; python_version == '2.7'
+scikit-learn
+scipy