[ci] Drop builds for Python versions we're dropping

We're decided that SWIG 4.0 will support Python 2.7 and >= 3.4.  The
patch to remove then (#1261) is waiting for the doxygen branch to be
merged, but meanwhile we're wasting a lot of travis resources on 6 CI
builds for Python versions we're not intending to support.

[skip ci]

.travis.yml

@@ -119,36 +119,11 @@ matrix:
       env: SWIGLANG=php VER=7.1
       sudo: required
       dist: trusty
-    - compiler: gcc
-      os: linux
-      env: SWIGLANG=python VER=2.4
-      sudo: required
-      dist: trusty
-    - compiler: gcc
-      os: linux
-      env: SWIGLANG=python VER=2.5
-      sudo: required
-      dist: trusty
-    - compiler: gcc
-      os: linux
-      env: SWIGLANG=python VER=2.6
-      sudo: required
-      dist: trusty
     - compiler: gcc
       os: linux
       env: SWIGLANG=python # 2.7
       sudo: required
       dist: trusty
-    - compiler: gcc
-      os: linux
-      env: SWIGLANG=python PY3=3 VER=3.2
-      sudo: required
-      dist: trusty
-    - compiler: gcc
-      os: linux
-      env: SWIGLANG=python PY3=3 VER=3.3
-      sudo: required
-      dist: trusty
     - compiler: gcc
       os: linux
       env: SWIGLANG=python PY3=3 VER=3.4
@@ -164,11 +139,6 @@ matrix:
       env: SWIGLANG=python PY3=3 VER=3.6
       sudo: required
       dist: trusty
-    - compiler: gcc
-      os: linux
-      env: SWIGLANG=python SWIG_FEATURES=-builtin VER=2.6
-      sudo: required
-      dist: trusty
     - compiler: gcc
       os: linux
       env: SWIGLANG=python SWIG_FEATURES=-builtin