Travis: Matrix Changes

This is not a beautiful way to change it, but it is an attempt at resolving issues raised in PR #243.  It results in 11 test.  Eight (8) test focused on testing Python 2.7 and 3.5 with many versions of Pandas.

.travis.yml

@@ -14,20 +14,41 @@ addons:
     - libgdal-dev
     - libspatialindex-dev
 
-python:
-  - 2.6
-  - 2.7
-  - 3.3
-  - 3.4
-
 env:
-  matrix:
-    - PANDAS_VERSION=0.15.2
-    - PANDAS_VERSION=master
-    - PANDAS_VERSION=0.16.2
   global:
     - PIP_WHEEL_DIR=$HOME/.cache/pip/wheels
     - PIP_FIND_LINKS=file://$HOME/.cache/pip/wheels
+  
+# matrix creates 3+4x2 = 11 tests
+matrix:
+  include:
+    # Only one test for these Python versions
+    - python: 2.6
+      env: PANDAS_VERSION=0.15.2
+    - python: 3.3
+      env: PANDAS_VERSION=0.16.2
+    - python: 3.4
+      env: PANDAS_VERSION=0.17.1
+
+    # Python 2.7 and 3.5 test all supported Pandas versions
+    - python: 2.7
+      env: PANDAS_VERSION=0.15.2
+    - python: 2.7
+      env: PANDAS_VERSION=0.16.2
+    - python: 2.7
+      env: PANDAS_VERSION=0.17.1
+    - python: 2.7
+      env: PANDAS_VERSION=master
+      
+    - python: 3.5
+      env: PANDAS_VERSION=0.15.2
+    - python: 3.5
+      env: PANDAS_VERSION=0.16.2
+    - python: 3.5
+      env: PANDAS_VERSION=0.17.1
+    - python: 3.5
+      env: PANDAS_VERSION=master
+  
 
 before_install:
   - pip install -U pip