dropping py2 support

.travis.yml

 language: python
 python:
-  - "2.7"
   - "3.6"
+  - "3.7"
 before_install:
   - if [[ "$TRAVIS_PYTHON_VERSION" == "2.7" ]]; then
       wget https://repo.continuum.io/miniconda/Miniconda-latest-Linux-x86_64.sh -O miniconda.sh;

README.md

@@ -8,7 +8,7 @@ prediction package with competitive accuracy and a fast and
 MHCflurry implements class I peptide/MHC binding affinity prediction. By default
 it supports 112 MHC alleles using ensembles of allele-specific models.
 Pan-allele predictors supporting virtually any MHC allele of known sequence
-are available for testing (see below). MHCflurry runs on Python 2.7 and 3.4+ using the
+are available for testing (see below). MHCflurry runs on Python 3.4+ using the
 [keras](https://keras.io) neural network library.
 It exposes [command-line](http://openvax.github.io/mhcflurry/commandline_tutorial.html)
 and [Python library](http://openvax.github.io/mhcflurry/python_tutorial.html)

mhcflurry/class1_affinity_predictor.py

@@ -422,7 +422,7 @@ class Class1AffinityPredictor(object):
                 numpy.testing.assert_array_almost_equal(
                     series.index.values,
                     percent_ranks_df.index.values)
-                percent_ranks_df[allele] = series
+                percent_ranks_df[allele] = series.values
             percent_ranks_path = join(models_dir, "percent_ranks.csv")
             percent_ranks_df.to_csv(
                 percent_ranks_path,