Switch Travis additional testing from python 3.6 to 3.7
diff --git a/.travis.yml b/.travis.yml
index 7b2b41c..8747913 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -159,7 +159,7 @@
       sudo: required
       dist: trusty
     - os: linux
-      env: SWIGLANG=python SWIG_FEATURES=-builtin SWIG_CC=gcc-6 SWIG_CXX=g++-6 CPP11=1 PY3=3 VER=3.6
+      env: SWIGLANG=python SWIG_FEATURES=-builtin SWIG_CC=gcc-6 SWIG_CXX=g++-6 CPP11=1 PY3=3 VER=3.7
       sudo: required
       dist: trusty
     - compiler: gcc
@@ -174,12 +174,12 @@
       dist: trusty
     - compiler: gcc
       os: linux
-      env: SWIGLANG=python SWIG_FEATURES=-builtin PY3=3 VER=3.6
+      env: SWIGLANG=python SWIG_FEATURES=-builtin PY3=3 VER=3.7
       sudo: required
       dist: trusty
     - compiler: gcc
       os: linux
-      env: SWIGLANG=python SWIG_FEATURES=-builtin PY3=3 VER=3.6 SWIGOPTPY3=
+      env: SWIGLANG=python SWIG_FEATURES=-builtin PY3=3 VER=3.7 SWIGOPTPY3=
       sudo: required
       dist: trusty
     - compiler: gcc
@@ -291,7 +291,7 @@
       sudo: required
       dist: trusty
     - os: linux
-      env: SWIGLANG=python SWIG_CC=gcc-8 SWIG_CXX=g++-8 CPP17=1 PY3=3 VER=3.6
+      env: SWIGLANG=python SWIG_CC=gcc-8 SWIG_CXX=g++-8 CPP17=1 PY3=3 VER=3.7
       sudo: required
       dist: trusty
     - compiler: gcc