BF: fix duplicate Python versions for --pre test

I stupidly changed the 2.7 to 3.5 when we were testing 3.5 in the next
matrix entry.

Author: matthew-brett

.travis.yml

@@ -54,8 +54,8 @@ matrix:
     - python: 3.5
       env:
         - DEPENDS="numpy git+https://github.com/pydicom/pydicom.git@master"
-    # test 3.5 against pre-release builds of everything
-    - python: 3.5
+    # test 2.7 against pre-release builds of everything
+    - python: 2.7
       env:
         - EXTRA_PIP_FLAGS="$PRE_PIP_FLAGS"
     # test 3.5 against pre-release builds of everything