ci: Attempt to fix travis failure updating python from 3.7.2 to 3.7.8

This commit is an attempt to fix the following error:

  [scikit-ci] Executing: python ../addons/travis/install_pyenv.py
  dyld: Library not loaded: /usr/local/opt/gettext/lib/libintl.8.dylib
    Referenced from: /Users/travis/.pyenv/versions/3.7.2/bin/python
    Reason: Incompatible library version: python requires version 11.0.0 or later, but libintl.8.dylib provides version 10.0.0
  /var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T/tmpkZ_cPw.sh: line 2:  2976 Abort trap: 6           python ../addons/travis/install_pyenv.py

Author: jcfr

.travis.yml

@@ -11,7 +11,7 @@ matrix:
     - os: osx
       language: generic
       env:
-        - PYTHON_VERSION=3.7.2
+        - PYTHON_VERSION=3.7.8
 
     - os: osx
       language: generic
@@ -20,7 +20,7 @@ matrix:
 
 cache:
   directories:
-    - $HOME/.pyenv/versions/3.7.2
+    - $HOME/.pyenv/versions/3.7.8
     - $HOME/.pyenv/versions/2.7.15
     - $HOME/downloads