Add Python 3.7 and 3.8 to Travis (#847)

diff --git a/.travis.yml b/.travis.yml
index 068e680..b57e9ef 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -97,7 +97,7 @@
     #  env: BUILD_SYSTEM=cmake C_COMPILER=pgcc CXX_COMPILER=pgc++
 
     ###
-    ## Python 2.7 and 3.6 builds on Linux
+    ## Python builds on Linux
     ###
     - os: linux
       language: python
@@ -122,6 +122,30 @@
           - gcc-5
           - g++-5
 
+    - os: linux
+      language: python
+      python: 3.7
+      env: BUILD_SYSTEM=python C_COMPILER=gcc-5 CXX_COMPILER=g++-5
+      addons:
+        apt:
+          sources:
+          - ubuntu-toolchain-r-test
+          packages:
+          - gcc-5
+          - g++-5
+
+    - os: linux
+      language: python
+      python: 3.8
+      env: BUILD_SYSTEM=python C_COMPILER=gcc-5 CXX_COMPILER=g++-5
+      addons:
+        apt:
+          sources:
+          - ubuntu-toolchain-r-test
+          packages:
+          - gcc-5
+          - g++-5
+
     ###
     ## CMake on OS X
     ##