Remove Python 2.6 on OS X from the Travis build matrix

Build times on these jobs have somehow crept up to nearly (and
occasionally surpassing) the Travis timeout (50 minutes). Additionally,
a look at the PyPI bigquery data indicates that 2.6 represents
approximately 0.4% of OS X downloads. Since this CI overhead is
causing the project significant pain for almost no gain, it's time to
eliminate it.
diff --git a/.travis.yml b/.travis.yml
index b45a4d4..b796617 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -53,10 +53,6 @@
           # 7.1 is OS X 10.10.x
           # see: https://docs.travis-ci.com/user/languages/objective-c/#Supported-OS-X-iOS-SDK-versions
           osx_image: xcode7.1
-          env: TOXENV=py26 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
-        - language: generic
-          os: osx
-          osx_image: xcode7.1
           env: TOXENV=py27 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
         - language: generic
           os: osx
@@ -82,10 +78,6 @@
           os: osx
           # 7.2 is OS X 10.11.x
           osx_image: xcode7.2
-          env: TOXENV=py26 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
-        - language: generic
-          os: osx
-          osx_image: xcode7.2
           env: TOXENV=py27 CRYPTOGRAPHY_OSX_NO_LINK_FLAGS=1
         - language: generic
           os: osx